Kim Burgess replied, I asked about getting the sfx under gpl, the answer was kind of: "No can do, I want to be credited!"
The GPLv3 allows requiring attribution
http://www.gnu.org/licenses/gpl-3.0-standalone.html#section7 but I don't think gpl2 does. =(
The following hint on how to produce fps gun sounds was also in the reply:
When it comes to first person weapon sound I usually find the most effect
way to approach the sound design is to visualize the individual actions
taking place and design the audio for that segment then merge and layer the
said assets either into a linear file or using real-time mixing in the games
audio engine. For example, the couchgun is a blackpowder firearm so there is
the initial click of the hammer shortly followed by a two blackpowder
explosions (there's two barrels, each firing a few milliseconds apart) which
is proceeded by reflections from the surrounding environment. To make it
more game/movie-ish you can compress the crap out of the explosion section
and add a bit of harmonic distortion to simulate the ears distorion of
sudden loudness.
