I think there's plenty of brownie points to be had for making sure a game runs well under Proton... There's very little reason that GoG cannot do what Steam does with Proton for Windows games.
That's not the same as a native client that launches games under Proton... GoG would be better served with something native/adaptive (even web/electrion/tauri based) that also installs/manages a Proton install along with the games.