My Conspyracy Theory about the motives of the Gyaru God:
My first thought about all of this is "Why did she reward the guy by turning him into a Gyaru?".
One could simply say "Duh, it's the Gyaru God, of course she sees that as a reward", but I'm not happy with that answer.
Gyarus all about just being themselves, not letting themselves be pushed by society, so why would the Gyaru God push someone that doesn't want to be a Gyaru, or even a girl, into being one?
What I think, is that being the Goddess of those that free themselves of societies expectations and just are their true and best selves (Gyarus), she can see inside people, see their true selves.
She uses that to help girls come out of their shell into the beautiful Gyarus they are inside, but too burdened under expectations, fears, etc.
Here, she would have seen inside our MC, and seen "his" true self, so she did what she always does: Let that be free, transforming the body to fit the true self.
But then she realized that turning a "boy" into a Gyaru, and especially one which such big anxieties and fears, would inevitable cause them a lot of problems and an identity crisis.
So she came back and is helping out by giving her a "Mission" of coming out of her shell and making friends, so she can come to accept her true self and get the happy life with lots of friends she deserves.