I think the issue with requests to "release the client" isn't as simple as "you can use an open source alternative".

Their Galaxy backend only handles Windows and macOS builds of games. Linux builds aren't included now. There are hacks around it like using access to individual files over HTTP through zip format for Linux installers as pseudo Galaxy (lgogdownloader supports that) but it's still just a hack.

Another piece is multiplayer integration that games can ship. That depends on their support too (authentication, matching and etc).