Unfortunately the Steamworks library updates we included to get access to the latest steam related controller tech seem to break Steamdeck. Why they would release a breaking SDK update without any notification of this remains to be seen.
The latest hotfix patch (1.3.0.3) available now adds a new launch option to the game “Play Grim Dawn (x64) - Compatibility Mode (SteamOS/Linux)”, which should get everything back up and running as before for those that need it.