I was able to install it pretty easily through the NonSteamLaunchers plugin for Decky Loader. That plugin even creates shortcuts with full art to launch in Game Mode, so it makes the process much easier than installing the game manually in Desktop Mode.
Download it in desktop mode, extract the zip to a folder. Then right click the .exe file and select "Add to Steam". Go back to gaming mode and select properties on the game, and make sure it uses a proton compatibility layer, or else it won't launch.
