Until any competing store releases a Linux client, I can't really argue against Steam. They are a gatekeeper and almost a monopoly, but they're also the most benevolent and pro-consumer gatekeeper that we have in the PC gaming distribution space. As long as all the competition continue to be Windows-only and, in some cases, actively work against Linux users, I don't want Valve's digital fiefdom to fall.
I'm not sure "gatekeepr" is the right term when all you do is simply being better for your customers than anyone else. Like, Ubisoft, EA, Epic, they all are garbage companies. GOG is the only store I'd mildly consider (ignoring tiny indie ones like Itch here), but they also have 0 interest in Linux support, which is where they lose me. Without Valve, Linux gaming would not be where it is today, and as a Linux user that is already like 85% of my decision making being done in favor of Valve - with the remaining 15% not all strictly being in another camp either. If someone wants to challenge that monopoly, they'd have to do something better than forcing exclusives or luring with "free" games, because that's some shady shit that makes me just want to stay away even more.
Valve isn't perfect though. Especially when it comes to owning games. I couldn't use Asprite on my laptop on my schools wifi because it couldn't verify that I own a 1gb program to draw pixels. Disabling wifi didn't help either. Still made up it's mind on not letting me make sprites for my school assignment till I connected it to my home wifi.
The best part? There's literally a free version that's not on steam that I purposely didn't download because I wanted to support the dev!
How are they a gatekeeper? Near monopoly sure. But they don't force companies to only publish on Steam. They don't have restrictive rules. I'm not sure what gate they are keeping.
If you reeeeally want to stretch, they do have rules about pricing things lower on other platforms. Like, you can have a sale on your website that makes it cheaper than Steam, but can't have the base price cheaper there than on Steam. That's about it.
You can't sell the free generated Steam keys on other platforms lower than on Steam. You are perfectly free to sell the game on other platforms for less than Steam.