I've won most of my games too and until today had barely even touched GR after I first got him. Today I was rotating between GR, CR, and Viking; just lost to Osiris with GR but then revisited with Viking and kicked his arse!
Point being there are lots of great characters in this game and it really is a combo of luck and strategy.
I think most of your wins going first are more due to the limitations of an AI defense as opposed to an overpowered GR, but that's just my opinion.