I love GOG and buy a lot of games through them, but at the same time one of the things Steam provides is very easy and simple online coop support. So if I ever intend to play with others I go with Steam. I don’t see an easy way around that.
i do wonder how hard it would be to integrate multiple of these APIs so that the end user could invite anyone from any platform, though i imagine you'll still need some sort of middle man / lobby server...
Either way, I'm not a fan of having to generate lobby codes every time I want to play baldurs gate 3 with my friends who have it on steam
Unless you've gone super deep into the Steam API, it shouldn't be too hard to plug in a different framework.