This might be really relevant for us, the Games Team, as there seem to
be quite a lot of games that have a different license for the engine
and the game data, and the combination of GPL and CC-by-sa seems to be
getting more and more popular.
And that is fine, so long as the game data is /also/ available under the GPL.
Note that it is not just games that have this problem, nor is this a problem specific to "content" vs "code". Many projects have incompatible licenses applied to different components or dependencies. Copyright holders often just need to be made aware of this legally makes the work undistributable as it cannot be distributed while in compliance with both licenses.
On the other hand, for some games (and theoretically for most of
them), the same game engine can be used with different data, and some
times vice-versa. If this is the case, the situation might be similar
to a media player and media data, or to a word processor and the
The case where the same game content can be used in a different engine+code verbatim is exceedingly rare.
As I understand the situation, what makes the game one copyrighted work is that the content is not generic, application-agnostic software as an image in the GIMP or a text document in a word processor. While the technical process that these are loaded may be very similar, this is an issue of legal judgement, not technical process. That the code and content is both software and is utilized as a single work is what makes them one work.
One could likely argue that if the game content were in a standard
format and displayed/interacted with much like a web browser displays
copyrightable work. An example of this may be ScrummVM. I'm not aware of any modern system like this, however.
Note that the conflict that's arisen with incompatible licensing is due to poor education on the GPL. Many people are under the false impression that the GPL only applies to executable code or that there is some problem with licensing content under the GPL. In truth, those looking for their work to be under the strong copyleft effect of the GPL are best not separating their work's copyright artificially as it may open a loophole for 3rd parties to release proprietary replacement game content.