I think GG is just fine. But if it were to be made a bit stronger, and I don't want it to be as it might unbalance things a bit, here's how I'd try to balance things.
GG gets stronger...
SSJ Goku gets chou KWave (just bigger, a bit more powerful than it is as SSJ now).
Gohan's Kame is already absolutely devastating as SSJ already, so no changes there.
Trunks would need a stronger burning attack, and make it a ball attack so he can get the hell out of there.
Cell and Frieza would probably be ok as they are as Cell would get a stronger GG too, and he of course has KWave. Frieza, 2 words, death ball.
Picolo just needs a more pumped Masenko.
Buu's probably also just fine as is.
I think that about covers it.
-Karrde-