Gamestop primarily sells games and Blockbuster primarily rented movies. These are two entirely different business models.
But, from an industry standpoint, there's a huge difference.
Blockbuster built itself up at a time when renting was the ONLY option. Movies cost $80+ and no one was going to pay that. Blockbuster was beloved by studios because it bought so many of their tapes. DVDs became dirt cheap and changed that, making people more likely to buy (why rent for $5 when you can buy for $7-12?) and making studios more interested in selling over renting. There still remained a pretty solid relationship between the two, though, as it was at least another revenue stream, even if a smaller one: Blockbuster buying DVDs still made sure DVDs got sold prior to the movie going on to cable and then networks, and stronger rental stats would help the studio sell broadcast rights for more.
GameStop pissed where it ate, so to speak. Game customers are more accustomed to buying instead of renting, so that revenue stream would always be there and games are budgeted expecting that. But GameStop moved primarily into selling used games. They'd undercut new prices by $5, which was less than 10% of a new game and they'd cannibalize new game sales. Studios got less, customers got slightly more, GameStop got significantly more. Studios were pissed off, and rightfully, that they lost such a huge chunk to a parasite.
So while they're two different business models, they're similarly hurting.
Plus why would anyone buy from GameStop used when you can typically get the same price from Amazon new?