Gamecuber isn't great English, but the language of videogaming is teaming with slang expressions - this just happens to be a near perfect word (and one that most people would understand, might I add!) for someone who plays the new nintendo Gamecube.
a golfer plays golf
a footballer plays football
a gamecuber plays the gamecube
I accept this doesn't work for all games though, but the reason is just to do with vowels being in the wrong places, i.e:
nintendoer
hockey'er