I worked for GAME for 4 years (2003-2007). You'll either be behind the counter (unlikely if you are under 18), on the shop floor, or in the back storeroom. If you're behind the counter you will have to make sure that games are rung up correctly, giving people trade-in prices for games, and you'll have to push the reward card. If you're on the floor you'll be keeping the shop tidy, advising people on games and consoles, and bringing out new stock. If you're in the back stores you'll be dealing with the deliveries and keeping the stockroom tidy.
The guys I worked with (and it really is vast majority guys, I was quite often the only female staff member in a store with 2 managers, 2 full-timers and 6 part-time staff) were always really nice and friendly, and knew their stuff about gaming. We always had at least one console in the back room to play games at lunchtime, though some managers took it to excesses (we had one manager who was a serious Pro Evo addict and would frequently be seen challenging the other staff).
All I'll warn you is that it will be difficult to get a job there as one who is under 18, both branches that I worked for (Tallaght and Dundrum) would never employ under 18s as they couldn't put them on the till to sell 18-rated games, though that could just be an Irish thing.
And in case you're wondering - yes, I am a massive gamer.