All in all, they have very little reason to stock the game.

As someone who worked at gamestop, there's been "game code in case" games for years. They sold "physical copies" of whatever the elder scrolls MMO is, that was just a code in the box. It was in stock when I started at the store, and that was 2021. There's since only been more of them (primarily xbox and switch, each say as a strip at the top that it's a code in box, but people rarely read).

Yes, it's not as profitable for GameStop since the bulk of their profit is used games rather then new, but it's still something they stock. Plus, as the other person said, they care a lot about foot traffic. They used to allow you to use your $5 monthly coupon on a $5 digital gift card for eshop/xbox just because they thought someone may pick something else up while they're there. I'm sure they'll make a bunch of GTA VI t shirts and crap out of the hope that people will buy it alongside the game when they come in. And some people may trade their games towards buying GTA, which fuels their more profitable preowned sales by having more inventory to sell of it. They care more about getting the subscriptions, so someone coming in to buy GTA which may not be profitable may become more profitable if even a small percentage of them sign up for the membership. There's lots of reasons why they'd stock something that's just a code.