[ja2ke]

EB Games (née Electronics Boutique), Software Etc, Funcoland, and international chains like Gamesworld were the competition, but they were all bought up by or merged with GameStop in the mid '90s through the '00s.

(edit: I'd also originally listed Babbage's in there with competition, but reading up reveals that GameStop was basically Babbage's, renaming themselves to GameStop at some point along the way as they were merging and consolidating.)

Other competition, now also deceased: Hollywood Video had GameCrazy for a while -- small gaming stores built into Hollywood Video rental stores -- which was actually a pretty spectacular game retail experience with great current and retro selections, but when video rental really started dying a few years back Hollywood closed them all down. I think Blockbuster had some game sales to try and compete as well, for a bit, but not to the degree of GameCrazy.

GameStop's only remaining competition is online with Amazon and digital distributors like Steam and OnLive, and in the (often superior to GameStop's) new game aisles of big box chains, probably most specifically* Best Buy and Toys R Us.

*(based on forums where people talk about buying games)