the G2A2 isn't that overpowered. Sure it's the best gun in the game, but it didn't stop me from doing some over 20 kill killing streaks with my penetrator.
So it isn't that overpowered, a little better than the penetrator but it's still more about the skills of the player. But yeah, most of the guys playing the multiplayer demo just sucked. (well actually almost all of them since I was in the top 3 like almost every round that I played)
I never really liked the G2A2 that much, it's the best one but I think it's kinda lame, the penetrator is a lot cooler, and so is the one with the scope and burst mode.