GameStop may be making money, losing money, or be breaking even. It all depends on what discount they offer GS resellers vs what discount they get from the companies whose gift cards they sell.
When you buy a $100 GS card from a reseller on ebay, GameStop does NOT get $100. They might get $90 or even only $85. Conversely, when you buy an ebay card at GameStop, it doesn't cost them $100. They get a discount on those as well. If they didn't get or receive a discount on gift cards, there would be absolutely 0 incentive to sell them.
Now, none of this takes into account the overhead GS takes on with the process. They also have to invest in inventory, floor space, and labor. So if they "break even", they're losing money, and even if they're coming out slightly ahead in terms of raw numbers, they may be losing some money as well.
JKalai just stated the same idea seconds ahead of me.