It has nothing to do with the source code licensing
While sources are licensed under GPL-3, the assets are not and the Amensia IP is still a property of Frictional Games
Distributing products based on others people IP is not something you can do on a whim, for a good reason
Good example is OpenMW - community built OSS engine for Morrowind. It requires user to provide own game files, and serves as "just engine"
You buy Morrowind, you download OpenMW, provide path to game files - and voila - game on a new engine.
This project could work like that, I think, if the author changes approach