GPL versus Creative Commons

GPL is the license under which most free software is released.
Creative Commons are a set of licenses under which most free art is released.
Both licenses can be used for collaborative development.

There are many Creative Commons licenses and they have only one thing in common: their name. The actual terms of licensing can vary broadly from one license to another.
See the article Richard Stallman gave to the LinuxP2P.com (link to web.archive.org):

Some Creative Commons licenses are free licenses; most permit at least noncommercial verbatim copying. But some, such as the Sampling Licenses and Developing Countries Licenses, don't even permit that, which makes them unacceptable to use for any kind of work. All these licenses have in common is a label, but people regularly mistake that common label for something substantial.

I no longer endorse Creative Commons. I cannot endorse Creative Commons as a whole, because some of its licenses are unacceptable. It would be self-delusion to try to endorse just some of the Creative Commons licenses, because people lump them together; they will misconstrue any endorsement of some as a blanket endorsement of all. I therefore find myself constrained to reject Creative Commons entirely.

I agree some Creative Commons licenses have nothing to do with Free Culture, but why is no one promoting the GPL license for artwork?

Here is what the Free Software Foundation publishes on their page on licensing:
Licenses for Works Besides Software and Documentation

• GNU General Public License
• GNU Free Documentation License
• Creative Commons Attribution 2.0 license (a.k.a. CC-BY)
• Creative Commons Attribution-Sharealike 2.0 license (a.k.a. CC-BY-SA)
• Design Science License (DSL)
• Free Art License