You can sell anything that's under GPL or any CC license EXCEPT ones with an "NC" clause, meaning non-commercial. For example, you can't sell Omega's military, which is under CC-By-NC if I remember correctly, unless he gives you permission. Of course, if you're the owner/creator of all the content in your mod, you can do whatever the heck you want with it (even if you also put it under an NC license).