Why do you think it needs an existing game to train the model on? They used Doom precisely because it already exists.

The entire point to the research paper was to see if humans could tell the difference between the generated content and the real game, that way they have a measurable metric of how viable this technology is even if only in theory, that means that they have to make something that's based off a real game.

Obviously the technology isn't commercially viable yet. But the fact that it looks even remotely like Doom shows that there is promise to the technology.