then some steam games would theoretically also be exempt because they don’t use steam drm.

I think the main difference that would arise between these and GOG would be the provision of installers. Even though some Steam games don’t use its DRM, they’re still reliant on Valve’s servers and an online connection for installation. GOG games are reliant on CD Projekt’s servers and an online connection for installer downloads, but upon download completion, one may install and reinstall games even while offline.

That’s a critical difference in digital distribution, in my opinion.