So, what sort of training did he undergo, exactly? We know a few things:
1) Was 8000 on Earth against Vegeta, 90000 on Namek when he landed (note: battle powers are peripheral to this discussion, I'd prefer to discuss the details of the training, but you guys can obviously discuss his intermediate PLs if they interest you).
2) Was comfortable with 2x Kaio-Ken on Earth (though used 3x, and 4x during the beam struggle; he also implied he could use 5x against Oozaru Vegeta), and seemed to feel he could use 10x Kaio-Ken after landing.
3) He was comfortable at 10x gravity on Earth, and by the time he landed, he had mastered 100x gravity (he trained at 20x and 50x on the ship along the way as well).
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Here are some relevant quotes from Herms (thanks!)...
Herms' Strength Checker wrote:---initially--- [#FR1#GOK#PRE]
Chapter: 251 (DBZ 57), 10.5
Goku: “If I don’t thoroughly toughen myself up from the basics…I won’t be able to withstand many-fold Kaio-Kens…!”Chapter: 264 (DBZ 70), P10.2
Context: Goku’s tired and can’t go on training.
Goku: “…But I conquered…50g…w-way faster than I…thought… M-maybe the duel with Vegeta…wasn’t entirely f-for nothing…”See AlsoDragon Ball: The 10 Best Training ArcsEpisode Guide | Dragon Ball Kai | Namek arcEpisode Guide | Dragon Ball Z | Namek arcChapter: 270 (DBZ 76), P8.3-4
Context: Narrator commenting on Goku’s increasing strength.
Narrator: “Goku had continued this insane pattern of pushing his body and ki to the brink of death, then refreshing himself again by eating a senzu. The 7 senzu he received from Lord Karin have already fallen to only 3…However, though this training is outrageous, perhaps Goku has somehow realized the Saiyan characteristic of their strength increasing when they overcome death…”---post-gravity training--- [#FR1#GOK#POS]
Chapter: 270 (DBZ 76), P11.1-4
Context: Goku has turned the gravity back down to 1g.
Goku: “Hwoo!!! I…I feel so light… …It’s like I’m not even here!!! Hoo hoo hoo!!! I’m light, I’m light! And strong, strong! Like this, I can definitely withstand about a 10-fold Kaio-Ken!”
Herms' History of Ever wrote:]December 18th, Age 762
Bulma and co. arrive on Namek
--Vegeta and his rival Kewi also arrive at about the same time. Vegeta begins a rebellion against Freeza, killing Kewi and then Dodoria. Meanwhile Gohan and Kuririn rescue a Namekian child named “Dende” from Freeza’s henchmen. Under Dende’s guidance, Kuririn sets off to meet the Great Elder of Namek. Goku departs for Namek himself, travelling in a remodeled version of the Saiyan spaceship that took him to Earth. Using the ship’s gravity control, he starts training in 30 and then 50 times Earth’s gravity. In the afterlife, Piccolo, Tenshinhan, Chaozu, and Yamcha arrive on the North Kaio’s planet and likewise commence training.
--DB Chapters 246-261 (DBZ 52-67) are set on this day.
[Ref: Date from Daizenshuu 7. Chapter 246 says Bulma and co. reach Namek 34 days after departing Earth, so this is that many days after Nov 14th. Telling when time passes on Planet Namek can be hard, since there’s no night. Everything up to Bulma and co. arriving on Namek to Goku talking with Kaio seems to be set on the same day (which means Goku reaches 50g on his first day of training!). According to Chapter 250 Goku leaves 2 hours after Bulma contacts Kame-sennin, and will reach Namek in 6 days, so that gives a limit to how much time can pass between now and then.]--DBZ Episode 50: Goku’s spaceship is endangered when it is captured by the gravity of Star αHZ. Goku unleashes a Kamehameha outside the ship, and the recoil gets him out of danger.
[Ref: Date from Daizenshuu 7.]
--DBO: Travelling from the far future, Towa and her Time Breakers attempt to interfere with the events on Planet Namek, but are tailed by the Time Patrol. Towa reanimates Kewi and Dodoria, experimenting on them to increase their size and power, but they prove unstable in their altered forms and are defeated by the Time Patrol.
[Ref: A description of one of the Time Machine Quests in the game.]December 19th, Age 762
--Freeza’s aide Zarbon beats Vegeta to the brink of death, but takes him back to Freeza’s spaceship to receive medical treatment when he learns Vegeta has hidden a Dragon Ball and is the only one who knows its location. At Freeza’s orders, the Ginyu Special-Squad interrupts its invasion of Planet Yardrat and heads for Namek to assist him.
--DB Chapters 261-264 (DBZ 67-70) are set on this day.
[Ref: Date from Daizenshuu 7. In Chapter 264 Freeza orders Ginyu and co. to come to Namek, and it’s said they’ll arrive in 5 days. Since Goku was going to arrive on Namek in 6 days, and he ends up arriving shortly after Ginyu and co., this means that a day has passed since Goku left. The transition into this day probably comes in Chapter 262 when the scene switches back to Namek after Goku’s conversation with Kaio.]
--Towa tries to interfere in Vegeta’s fight with Zarbon, experimenting on Zarbon to increase his size and power. Her experiments again go awry, and the altered Zarbon is defeated by the Time Patrol.
[Ref: A description of one of the Time Machine Quests in the game.]December 20, Age 762
--After recovering from his injuries, Vegeta escapes Freeza’s ship with the Dragon Balls. Kuririn arrives at the house of the Great Elder, who draws out his dormant power and entrusts him with the Namekian 1-Star Ball. Zarbon tracks Vegeta down and fights him again, but Vegeta is stronger now and kills Zarbon. Kuririn returns to Gohan and Bulma, then sets out again for the Great Elder’s house with Gohan in tow so that the Great Elder can draw out his power as well.
--DB Chapters 264-270 (DBZ 52-76) are set on this day.
[Ref: Date from Daizenshuu 7. In Chapter 264 the narrator explains about how Namek has no night, and says about 1 day passes, leading to this day.]December 22nd, Age 762
--In his spaceship traveling to Namek, Goku collapses while training in 100 times Earth’s gravity, but uses a Senzu to recover.
--DB Chapter 270 (DBZ 76) is partially set on this day.
[Ref: Date from Daizenshuu 7. In Chapter 270 as the scene shifts to Goku’s training, the narrator says “2 days later” (Viz changes this to “meanwhile”). The narrator also says Goku has been in space 4 days, so this is that long after the 18th.]December 23rd, Age 762
--Goku completes his 100g training in his spaceship. He decides to rest in the remaining time before he reaches Namek.
--DB Chapter 270 (DBZ 76) is partially set on this day.
[Ref: Still in Chapter 270, from that last part with Goku’s training, the narration says that another day passes.]December 24th, Age 762
The battle with Freeza
--The Ginyu Special-Squad lands on Planet Namek and begins fighting Vegeta, Gohan, and Kuririn. Goku arrives on Namek soon after, and the Ginyu Special-Squad is defeated. Goku is heavily injured in the battle, and is placed in a medical machine to heal for about 40~50 minutes. During that time Kuririn and the others use the Namekian Dragon Balls to summon Polunga, wishing Piccolo back to life and by extension reviving God and Earth’s Dragon Balls. Vegeta, Gohan, Kuririn, and the revived Piccolo (brought to Namek with another wish) battle Freeza, but are outclassed. Vegeta is killed, while Goku finishes healing and confronts Freeza. Ultimately Goku transforms into a legendary Super Saiyan and defeats Freeza, but their fight destroys Namek. Using Earth and Namek’s Dragon Balls, all those killed by Freeza and his henchmen are brought back to life and transported safely to Earth, while Goku escapes the exploding planet with one of the Ginyu Special-Squad’s ships.
--DB Chapters 270-329 (DBZ 76-135) are set on this day. Manga-wise, this is the “longest” day in the series, making up 60 chapters and about 11.5% of the series’ total run.
[Ref: Date from Daizenshuu 7. In Chapter 250 it says Goku will reach Namek in 6 days, so this is that long after the 18th. In Chapter 270, after Goku’s training scene, Freeza says that Zarbon has been gone for 4 days, and Kuririn says it’s been 4 days too, so this is also 4 days after the 20th. Goku healing for 40~50 minutes is from Chapter 291. These events occurring on Christmas Eve may be a reference to a short (5 panels) side manga by Toriyama featured in Weekly Jump No.3-4, 1991, which shows the characters celebrating Christmas in various ways, all where they would be during this point in the Namek/Freeza arc.]
—DBZ Movie 3: Tullece and his men plant the Shinsei-ju on Earth, planning to eat its fruit once again and achieve power rivaling Freeza. They are defeated by Goku, and the Shinsei-ju is destroyed.
[Ref: While noting its inconsistencies with the main series and calling it a “movie-only story”, Daizenshuu 6 says this is between Goku’s arrival on Namek and his showdown with Freeza, both events which are on December 24th.]--DBZ Movie 4: Slugg and his Demon Clansmen attack Earth, trying to convert the entire planet into their personal spaceship, but Goku defeats them.
[Ref: While noting its inconsistencies with the main series, Daizenshuu 6 places this between Goku’s arrival on Namek and his transformation into a Super Saiyan, both of which are on December 24th.--DBO: Towa alters Recoom, but like before the Time Patrol defeats her experiment. Towa next has her Time Breakers try to exterminate all Namekian frogs in the region, to prevent Ginyu’s defeat. She plans to take control of Goku’s body while Ginyu’s soul is inside. With Bardock’s help, the Time Patrol foils her plan, defeating the gigantic frog-Ginyu Towa sends after them.
--DBO: After Nail’s battle with Freeza, Towa steals some of his Namekian ki. She infuses it into Freeza’s severed tail, which she feeds to one of Freeza’s henchmen, making him grow in size and power. The Time Patrol defeats the henchman, only for Towa to enhance Ginyu’s body as well, but with similar results. Her various experiments a failure, Towa returns to the future.
[Ref: A description of two of the Time Machine Quests in the game.]
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I think there are a few talking points here:
1) How many zenkais did Goku have total? The quote about 7->4 senzu is curious...is the senzu Karin gave him in the hospital included in that 7 (I'd say no)? Also, did he give Gohan and Kuririn whole senzus on Namek, or split one in half?
I'm assuming the senzu in the hospital is not included in the 7, and Gohan/Kuririn each got whole senzus, in addition to Vegeta's. So he ate 1+7-3=5 total, accounting for 5 zenkais between the fight against Vegeta and his arrival on Namek.
2) What kind of training did he do each day? The trip took 6 days total, so without rereading the manga to flesh out the list, I think it's something like:
Day 1 (12/18) - starts at 20x gravity, speaks to Kaio (at the end of the day, according to Herms); originally says he wants to go from 20x->30x, but after hearing about Freeza decides to jump to 50x (Herms has him training at 30x and 50x gravity; I'll have to reread) [c251] [c258] [c260] [c261]
Day 2 (12/19) - presumably training at 50x gravity, we get to see one panel [c264]
Day 3 (12/20) - training at 50x gravity [c264]
Day 4 (12/21) - takes final senzu after getting hit by his own Kamehameha (maybe? or perhaps this is Day 5...slightly unclear) [c270]
Day 5 (12/22) - finishes training at 100x gravity, does not eat a senzu [c270]
Day 6 (12/23) - rest
There aren't any relevant scenes between 251-258 (Freeza attacking village/Dodoria stuff) or after 271. Now, it seems the bulk (perhaps all of?) of his training is, as he planned, getting used to the gravity and working on his fundamentals, to build up his Kaio-Ken to higher multipliers. I'm mostly wondering:
• Did he actually use Kaio-Ken on his ship at all?
• When did he take the other three senzus not accounted for in the manga (perhaps there are suggestions in the anime)?
• When did he transition from 50x->100x gravity (again, are there any suggestions in the anime)?
• At what point did he become more comfortable with higher levels of Kaio-Ken?
3) What were the other benefits to the training, other than building up his Kaio-Ken? Obviously he increased his battle power using zenkais, and his speed/durability improved, though I'm not sure if those were just proportional to his ki.
Apologies for the long post, I was just playing my old Goku Gekitouden cart recently (the battery is shot though after all these years, so saves don't always stay, heh), and having enough fun with the Goku training mini-game (pretty fun) that I decided to skim the manga and post this. 
