So I decided to purchase The Falconeer on GOG a little while ago. And I attempted to Install the game on my Steam Deck using the Offline Installer. The main game was no problem, simply run the package in Proton and redirect to your desired Install location. The problem comes with the DLC. The offline DLC doesn't allow you to install anywhere except the default installation location. This is a major problem on Linux, as the C/ drive doesn't exist.

For those of you who say "Use Heroic Launcher" I found an issue with that. It seems Heroic launcher doesn't like large file size games. An attempt to install a game over 10 GB, in my case Batman Arkhan City, the downloader will stall partway into the download, forcing you to cancel.

Now I did find a work-around, but it's a bit time consuming, and requires extra hardware. I got out my old Windows laptopb and installed the game from GOG Galaxy to an external drive, then copied the installation folder to my steam deck. This is not preferable as it requires a Windows device. You could call it a form of DRM.

This also extends into extras like Soundtracks and concept art, as those also use installers that require the defualt install location. Although I'd suggest to GOG that Standalone Extras be downloadable in their raw file formats rather than an .exe installer. I'd rather have the soundtracks directly on my phone anyway.

...

I've never heard of soundtracks and art books in an installer package. Open a support ticket and try contacting the developer, because that is completely ridiculous. Did you try something like innoextract first? For a site focused on DRM-free gaming, surprisingly few people here have the faintest idea what DRM is. No, a game being made for an OS you're not using is not DRM.