As onpon4 wrote, the copyright law and the trademark law are completely
different. The GNU GPL is a software copyright license.
The artwork is not "linked" to the GPL code and can bear whatever license,
i.e., the GPL is not mandatory for it. Warsow is a famous example: the code
is GPLed but the artwork cannot even be freely shared... what is bad! Yes,
artwork need not be free but one should at least be allowed to
non-commercially distribute it.
Here is an interesting reference about rms' opinion on this specific issue.
