They do provide Linux support in other ways though. They even troubleshoot me once with a game I tried to play on Linux and offered a refund.

Gog Galaxy not on Linux is a shame, yes, but its DRM-Free and Linux installers are enough for me to continue to buy from them.

Edit: Heroic Launcher makes a great replacement of Gog Galaxy, maybe even better than the Windows client, from what I'd tried. No multiplayer though.