poVoq like writing a mod for a game and releasing the source-code to it.
Yeah that comparison makes sense to me. But this means libre games for Windows would be questionable, too.
@ffaf Good point about attracting people to libre software - be it Windows or a proprietary engine. I am not sure but I guess the Unreal Engine would not be neccesary or useful in this way. So it seems poVoq is right.
Edit: No there is no GPL for UE.