I have to admit it may sound funny how I’m willing to accept the way Blender people interpret GPL on a certain matter while not on another but I’m pretty positive that his answer on this matter is inaccurate.
The key question is whether or not the assets are a part of a single combined program (i.e. the game) or just data. As long as we see them as the latter, it should not matter how we distribute them.
Read these parts from the official FAQ about the GPL license:
…copyright law does not give you any say in the use of the output people make from their data using your program. If the user uses your program to enter or convert her own data, the copyright on the output belongs to her, not you.
The output of a program is not, in general, covered by the copyright on the code of the program. So the license of the code of the program does not apply to the output, whether you pipe it into a file, make a screenshot, screencast, or video… Keep in mind that some programs, particularly video games, can have artwork/audio that is licensed separately from the underlying GPLed game.
As far as I see, this is the ground on which the Blender foundation sees .blend files (without scripts) as program output thus exempt from the GPL requirements. While this interpretation may be questionable to some, as long as we accept it there should be no such restriction GPL does not mandate anything on program output or data.
And GPL even allows distributing differently licensed software binaries in the same media (i.e. archive, DVD, etc.) if they form an “aggregate” rather than a “combined whole”:
An “aggregate” consists of a number of separate programs, distributed together on the same CD-ROM or other media. The GPL permits you to create and distribute an aggregate, even when the licenses of the other software are nonfree or GPL-incompatible.
In other words, there is no reason why you should separate assets in a different distribution from your GPL scripts when publishing an UPBGE game as long as we can see them as “program output” or “data”.
Hope this clears up the confusion. And I think the fact that even UPBGE’s official manual and no other person than the one who created Blender gets confused about the license shows why we need to improve our documentation on this matter so that the end users have a clear idea as to how to make a non-GPL game with UPBGE.