Youdon't need permission
you do need permission. and you have some permissions (the ones described in the GPL), which are given by everyone who distributed the game.
You dont. The license grants full rights to re-distribute, including for a price. Guess most of you arent really acquainted with the gpl license. And i guess the gpl hasnt really been tested until now. in the case of tremulous, not only the source code, but the data files, and the name itself are under the gpl, which makes pretty much everything redistributable even with a fee. The only requirement is that source code is provided. The GPL is about freedom to copy, not about ensuring nobody makes dough with it.
You can all ready up more on the Free software foundation webpage.