I think they typically give 1/3 to 1/4 of what they resell your game for, it's not a lot. You are better off finding another way to sell your games, like Craigslist or Amazon.com. You can also buy games there for cheaper than Gamestop prices. You just have to be careful. Don't buy from eBay, that site is overrun with bootleg copies that will only cause you grief.
A good trick I have used once is to catch someone trying to seel his stack of games at Gamestop. Tell him you will give him $1 per game more than Gamestop will pay, and you will give cash instead of store credit. Then let him price them out at the counter, pull out your wallet, and buy the lot. At worst you can try them and then sell them at the same store and take a $1 per game loss for the fun of trying them.