GFN works with anything that can be installed at Nvidia’s end. The only conceivable impossible thing to support would be software that married itself to your hardware or uses dongle functionality. There was a time when Nvidia just put any game they felt like up on the service like it ain’t no thang. They had just about every Bethesda game, for example, and were forced to take them down. Or, if you prefer - asked nicely.
It’s more opaque on other platforms, but on Steam if you are a developer, all you have to do for your game to be on GFN is tick a box in a menu. There is no work to do at all. If a game is available to stream anywhere, but not on Nvidia, someone has an exclusivity deal, or the publisher has some other reason to prefer directing users to a different service. In this case, Diablo 4 players on Battle.net who bought the game before it came out on Steam, represent a huge potential customer base for Game Pass. The Steam version is probably only streamable in order to sell copies, and because the Steam community is full of people who have no reason to be on game subscription services.
Meanwhile, Bnet D4 supposedly is “coming soon” to GFN along with other Bnet games, delayed indefinitely for untold reasons. Since it can already be streamed on Boosteroid and AirGPU, my guess is that Microsoft simply wants something in return, or perhaps Nvidia has taken inspiration from publisher stubbornness and expects a cut of microtransactions made possible by their service. This will become more interesting as an increasing number of people prefer paying $5 a month for streaming, rather than spend hundreds or thousands of dollars on upgrades and periodically a new rig.
Remember though that Microsoft is only on GFN at all because of an agreement reached in court. But so far as I can tell, they have not agreed to bring all of their games to competing platforms, and they might even be doing it without formal agreement, to show good faith before having it forced on them.
I have said it before and I will say it again. The US Congress passes some 300 laws per year on average, very few of them involving consumer protections, and very often NONE of them focused on protecting consumer rights in the world’s biggest entertainment industry. This needs to end. Gamers have an old reputation for libertarian views and grassroots victories versus powerful organizations; and gaming carries a unique loser stigma not suffered by people who spend most of their time reading, watching television, or playing with smartphones. This reputation is outdated. Our culture is directed by paid influencers, our advantages versus corporate power are none, and our social isolation has long since been surpassed by app-addicted mainstream society. Yet the notions that our concerns are important and our rights need government protection continue not to be taken seriously, even amongst ourselves. That needs to end before we can hope to be treated with the same respect as other consumers.