Comment by rpdillon
Their DRM seems to be okay, but they do have some weird bugs.
My biggest gripe with Valve right now is that I bought a copy of No Man's Sky on GOG, and then I also had a copy on Steam. And so I let my son play my Steam copy through Steam Library sharing so we can play co-op while I play my GOG copy. Unfortunately, because I launched my GOG game through Steam, Steam's DRM won't let him play at the same time as me because they think we're playing the same copy.
It seems to be that they simply look at the title of the game and or the executable name to figure out what game it is, but they don't check to see what storefront it was bought from. I'm not sure about this though, I have to do more investigation.
In case you launch the GOG game because of Proton, then I suggest using Heroic launcher to start it instead. You can use Proton there too, automatically downloads and everything, same as Steam. And there will be no clashes with Steam.