They are mostly games I just buy and trade back in. Right now this weekend, they bumped up trade in values AND have a promotion for 50% more trade in credit when you purchase the Doom and the Uncharted 4 game. I usually just buy the game, ask for a gift receipt, and return it back on my gift card. For instance, right now the Rise of the Tomb Raider for the 360 is $20 at
Best Buy with their gamers club unlocked membership, and you can trade it back into gamestop for $40 with the promos. Same with NBA Live 16 (bought at Fry's for $12.99 and traded it back for $33). Some of the games I traded in yesterday are from my own collection that I just don't play anymore too.
Also... I was at the mall today, went to the Gamestop I normally don't frequent and checked for Ebay cards. They didn't have any, so I asked the employee If I could consolidate my gift cards because "it's leaving an indent in my wallet"

The employee combined the rest of my gift cards onto one. She basically bought 1 $500 gift card with my 5 $100 gift cards and it went through just fine. I was asking her a bit more about the gift cards and she said you can put a max of $500 on a gift card AND you can purchase them digitally and physically online. I haven't yet checked if the website GC > GC purchase works, but it's a great option if you want to stay low key/don't want to look weird swiping 5 gift cards at the register.