What's this bullshit about Tremulous being under the GPL?
The code, binaries, and qvms are licensed under the GNU General Public License 2. That license is about publicizing software. It says that you may not change the license, and if you make modifications to the software, and distribute it, you must provide the changes in source form too. There is not a single word about using the content commercially, and "selling" the code is explicitly allowed: "You may charge a fee for the physical act of transferring a copy, and you may at your option offer warranty protection in exchange for a fee."
Textures, maps, models, animations, sounds, music, etc. are licensed under the Creative Commons Attribution-ShareAlike 2.5 license. That license is used for non-programmable art. As with the GPL, you can make money with CC'd content.
I don't know what to say about licensing art under the GPL (is it even possible?). Since the GPL states that you must include sources to all your distributions, if you release maps under the GPL, you must also give its sources out too.
Also, a lot of map .pk3s don't contain any licensing information, which means that you are allowed to use it for whatever you want. If the file was at some point distributed with a license, I believe it is the liability of the entity that broke the chain.
Then the Devs should not have used the GPL license.
That is just plain invalid! Nobody has "chosen" that license. People can borrow GPL'd and CC'd content, which is a opportunity for the Devs to create something. It's not that the Devs decided to give their own work out like that. If they owned the content, they would have permission to rerelease it under any license.
What about the BSD license? It's 2 sentences long: you may not remove the copyright, and you must reproduce the copyright. (Interpretation: if you redistribute copies of the software, please give the author some credit.) And the BSD guys DO provide polished code for free and their intent is to make it available for commercial use.PS: Knowledge is power and is to be respected. If someone does not know anything about copyleft licenses, then let them pay for Tremulous.