IT wasn't in the damn manga, therefor is FILLER.Yep, that's basically the only solid evidence there was. Vegeta explains that the last SSJ had to transform into the Oozaru state in order to control his power. Unfortunately, the Saiyan couldn't control the Oozaru state, so he went on a rampage and destroyed the planet he was on.
This is the Oozaru you see while Vegeta was explaining the last SSJ:
therefor it isn't Canon.
it was made by Toie Animation because they had ideas of GT before DBZ was done. As you know ,they added that because Akira was supposed to end it at frieza, where goku turned supersaiyan. Then they wnted something else, but Akira's editors wanted him to continue so They made GT at the very end, including the Golden Oozaru.
Gigi aswell is not Canon, therefor its not really a transformation