Here, we enter the first series of Dragonball GT...

Ten years passed since the end of Dragonball Z. Ubuu trains with Gokou. Goten and Trunks are adults. Pan is 14 years old. Sataan is balding. Klilyn has a moustache and grey hair. Since the defeat of evil Boo, there was peace on earth...
Pilaf has finally gathered all the Dragonballs to wish for world domination. Shenlong is summoned. Without notice, Gokou enters the scene. Pilaf, surprised to see Gokou after all these years, starts to wish that Gokou was still a kid so that he could be easier to defeat. Shenlong takes this for the wish and turns Gokou back into a kid. Pilaf runs off again and is never seen again.
Gokou returns to his house and tells about what happened. PoPo appears and tells Gokou that there are Black Star Dragonballs scattered in space. Gokou decides to find these Dragonballs to restore himself to his normal form. (The Dragonballs of the Earth can only be used to grant wishes once a year.) More surprisingly, PoPo tells that these Dragonballs must be gathered within a year or the Earth will explode! Gokou and Trunks get on a space capsule to set voyage on their new quest. Pan sneaks aboard the capsule and accidentally damages an engine. The space capsule crashes into many planets. In these planets, our heroes acquire the Dragonballs and meet a cast of minor characters. During these scenes, Gokou and Trunks enter their Super Saiyan form for the first time in Dragonball GT to battle the minor villains. Pan also fights, but spends most of her time watching the Super Saiyans in amazement. In addition, our heroes meet Gill, a little robot who swallowed the Dragonball radar. As a result, our heroes take him on their journey.
Our heroes land on Dr. Myuu's planet. Dr. Myuu is an evil scientist who creates a bunch of cyborgs and robots. Gill is one of his creations. Trunks is kidnapped. Pan tries to rescue Trunks while Gokou fights Ruudo, a creation of Dr. Myuu. Ruudo is killed. Gill helps Trunks and Pan escape and our crew returns to space. One of Dr. Myuu's project is Baby. Baby is really the only surviving Tsufuru-jin. Planet Plant, his native planet, was destroyed by the Saiyans back when the Saiyans worked as mercenaries for Freeza. As the only survivor, Baby wants revenge on all Saiyans, innocent or guilty. Dr. Myuu found Baby in space and saved him. Baby has the power to take over bodies and acquire his victims' power. Baby takes over Dr. Myuu's body and kills him in the betrayal.
Gokou, Trunks, Pan, and Gill finally return to Earth with all seven Black Star Dragonballs. Gokou takes them to PoPo to secure them for later use. However, Baby also lands on Earth. Slowly, he takes over the bodies of Trunks, Gohan, and Goten.
Even when he leaves their bodies, an egg is left in their brain. This egg makes its victim a zombie of Baby. (When Gohan was Gohan-Bebi, he fought and injured Piccolo with a fireball.) Baby finally finds a body that satisfies him- Vegeta's! Vegeta's body was taken over and Vegeta-Bebi is formed! Soon, everyone on Earth become zombies of Baby except Sataan, Pan, Buu, Ubuu, and Gokou. PoPo, also a victim of Baby, uses the Black Star Dragonballs to create another Planet Plant for Baby.
Gokou faces Vegeta-Bebi. Vegeta-Bebi is stronger and easily defeats the young Gokou. Gokou powers up to Super Saiyan level 3, but is still defeated; Vegeta-Bebi can also power up to level 3. Pan hides in the background and watches the fight. The defeated Gokou is pulled out of the fight by Kaiou-shin. On Kaiou-shin's planet, the Kaiou-shin inhabitant substances are creating a formula that will make Gokou's tail grow back. (The tail is the main source of power for a Saiyan.)
On Planet Plant, Ubuu faces Vegeta-Bebi and is defeated. Fat Buu joins the battle and is killed. Buu's body is near Ubuu and his spirit enters Ubuu. The two fuse and Ubuu learns that he and Buu were originally one. However, even with this fusion, Ubuu is no match for Vegeta-Bebi. Ubuu tries to change Vegeta-Bebi into a chocolate but his plan backfires and he turns into a chocolate which Vegeta-Bebi eats.
Gokou's tail is only partially grown. Kaiou-shin pulls it to make it reach normal length. Gokou returns to planet Plant to face Vegeta-Bebi. After getting beaten by Vegeta-Bebi again, Gokou lies on the
ground and stares at the Earth. The "full-Earth" acts as a full-moon. As a result, Gokou reaches Oozaru stage and becomes a giant golden ape. Gokou is a lot stronger, but is unable to control his body in Oozaru stage and trashes everything in sight. Pan helps her grandfather remember who he really is and another change happens. Gokou acquires control over his body and enters another stage beyond Oozaru. He powers up and becomes Super Gokou 4. The tide of the battle is turned and Vegeta-Bebi is losing. The zombie Bulma creates a special magnifying glass for her husband. It magnifies the rays of the full-Earth and Vegeta-Bebi enters Oozaru stage.
Gokou gets defeated once again. Meanwhile, Kaiou-shin resolves that Chou-sui water could cleanse people from Baby's control. He goes to Sky Palace to get the Chou-sui water. Dende and PoPo are cleansed in the struggle against Kaiou-shin.
This water is spread to everyone. Gohan, Goten, Trunks, (now free from Baby's control) and Pan surround Gokou and form a circle. Their energy is donated to Gokou. Gokou and Vegeta-Bebi face each other again. Gokou does a Kamehameha fireball and defeats Vegeta-Bebi. Baby leaves Vegeta's body and runs to a space capsule to escape, but is killed.
The Earth is about to explode because the Black Star Dragonballs weren't used to wish for the Earth's stability. Our heroes fly all the inhabitants to planet Plant. Piccolo decides to stay on Earth to die with it because he loved it so much. (He is still injured from the blast from Gohan-Bebi.) The Earth explodes, and Piccolo goes with it. At The Namek Dragonballs are gathered and the Earth is restored from the wish, but Piccolo is still dead. However, one thing is learned- Gokou reached a new stage as
Super Saiyan...

PS: Super Gokou 4 level is not the same as Super Saiyan 4 level. Super Saiyan level is characterised by the golden hair.
Super Gokou 4 level is what resulted in the power-up from Oozaru (giant ape) form.

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