Yeah for a while now I’m been buying games on GOG where possible and keeping an archive of them, because I know at some point every company will eventually let you down.
The GOG launcher is optional (I don’t use it). On their website you can download installers for every game you own, and these installers don’t require the GOG launcher, or even a network connection.
Honestly I’m not sure - as I said I don’t use it. I know of at least one game that’s “DRM-free” but requires the GOG launcher for multiplayer (No Man’s Sky). That’s fairly controversial and I think the only reason why it’s on GOG is because it came onto GOG back when it was a singleplayer only game.