So ever since the March 8 2025 update of the mod the game has been broken.

What fixed it for me on Deck was:
Sub to mod.
Launch game trough mod launcher.
Get an error about renderer.
Re-launch mod launcher.
Scroll to GD3D11 and click on it.
Select "Mod files" button in the toolbar of the launcher and a win 95 looking file manager will pop up.
Go into the System folder and delete everything.
Download an older version of the mod from github and extract it somewhere. I used 17.8-dev9, but dev19 has also been confirmed to work.
Copy/paste the files from github to the System folder. Note, that it will paste all the files from the GD3D11 folder in the System folder and not in the GD3D11 subfolder. Dont do anything about that, just close the file manager and re-open it from the Mod FIles button in the mod launcher and you will now see it has created the folder and all the files have been moved inside.
Select Gothic in the mod launcher and press the Play button - your game should work now, with the mod enabled.

Props to steam user Stoxy who figured it out.

Do the file deletion and replacing trough the pop-up file manager and not trough steam OS desktop mode, as that does not seem to work.

Optionally disable wi-fi to prevent steam from updating the game files with the broken mod version.