![super saiyan god aura super saiyan god aura](https://cdnb.artstation.com/p/assets/images/images/016/500/641/large/kris-robinson-super-saiyan-god-by-krizekko-dd1wwnu-pre-2.jpg)
Despite the name, it is not related to the Super Saiyan form and exists as an entirely separate line of transformations. So, why is Goku using SSG, probably because its his most powerful form that doesn't cut his power in half after a minute of use. Super Saiyan God (, Sp Saiya-jin Goddo), occasionally referred to as Super Saiyan Red (, Sp Saiya-jin Reddo) due to its hair coloration, is a divine transformation unique to the Saiyan race. His attacks had all the power of Blue, but the stamina drain of Blue was only in effect for an instant at a time, so he could maintain his max power for longer. Red drains far less stamina than Blue, and so he used Red which he had mastered (He managed to contain the aura like Goku did for Blue), and then would transform into blue right as he attacked for an instant.
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He then revealed that that was his answer to the stamina problem. In chapter 22, around page 15, Vegeta transformed into SSG, the Red form. Vegeta's solution to this was displayed a few chapters earlier.
#SUPER SAIYAN GOD AURA FULL#
On the same page as the quote I just said from Vegeta, he also noted "the fact that its only able to output full power for a very short time is blues greatest weakness". Vegeta had a different solution, one that I think addresses the question quite nicely, albeit indirectly. He was able to fight on near equal footing with Fused Zamasu for a while at least. As Vegeta put in, he was "fighting at a constant 100% of his power". Goku replicated this with SSB, and so it can drain little stamina to keep active. By containing that aura in their body, they reduced the stamina drain down to almost nil, and so were able to stay transformed for extended periods of time. The aura they release costs them power, draining stamina. This is exactly what what he and Gohan did in the Hyperbolic time chamber for SSJ 1. Goku, in chapter 24, around page 33, sealed his power in his body. Vegetas in particular is what we want, but understanding both is important. They demonstrated these in their fight against Zamasu. In the Manga, Goku and Vegeta both thought up methods to conserve stamina when using SSB.