Well, just to clarify, the “derivative works” clause only applies to runtimes. If you keep the .blend file separate, you still retain all rights over the game as your individual copyrighted work.
Taken from the GPL, Section 0, Paragraph 2:
The act of running the Program is not restricted, and the output from the Program is covered [by the terms of the GPL] only if its contents constitute a work based on the Program (independent of having been made by running the Program). Whether that is true depends on what the Program does.
Individual .blend files cannot be called derivative works, but rather, they are the product of the program. However, all runtimes made with any version of Blender later than 2.25 are considered derivative or collective works. (Versions 2.25 and previous are not licensed under the GPL, but rather under the old NaN EULA, which allows runtimes to be created and copyrighted soley to the game creator.)
The GPL, Section 2, Paragraphs 2 and 3 (emphasis added):
These requirements [from Section 2, paragraph 1] apply to the modified work as a whole. If identifiable sections of that work are not derived from the Program, and can be reasonably considered independent and separate works in themselves, then this License, and its terms, do not apply to those sections when you distribute them as separate works. But when you distribute the same sections as part of a whole which is a work based on the Program, the distribution of the whole must be on the terms ofthis License , whose permissions for other licensees extend to the entire whole, and thus to each and every part regardless of who wrote it.
Thus, it is not the intent of this section to claim rights or contest your rights to work written entirely by you; rather, the intent is to exercise the right to control the distribution of derivative or collective works based on the Program.
As far as legality goes, this holds water in court:
The GPL, Section 5:
You are not required to accept this License, since you have not signed it. However, nothing else grants you permission to modify or distribute the Program or its derivative works. These actions are prohibited by law if you do not accept this License. Therefore, by modifying or distributing the Program (or any work based on the Program), you indicate your acceptance of this License to do so, and all its terms and conditions for copying, distributing or modifying the Program or works based on it.
All GPL’ed files are copyright to their respective creators (BlenderPlayer is copyrighted to the Blender Foundation), and any and all use outside of the terms of the GPL constitutes a violation of copyright law.
I suggested a potential solution to the runtime inherited GPL problem a while back in the News and Discussions thread. I’m not sure if the GPL Linking Exception was intended to be used in that way, though.