A caveat:
The catch is, once you put in the GAU8 sound file, you have to rename it to one of the default gun sound names in order to activate it -- its the only way to use it. Your choices are .303 cal., 7.7 mm, 12.7 mm (.50 cal), 13 mm, 20 mm, 30 mm and 40 mm. The associated sound files are located in the main SOUND folder as:
13GUN.WAV (13 mm)
20CAN.WAV (20 mm)
30CAN.WAV (30 mm)
40CAN.WAV (40 mm)
50GUN.WAV (12.7 mm or .50 cal.)
303GUN.WAV (.303 cal.)
792GUN.WAV (7.7 mm)
These are the names you will choose from for renaming and are also the default choices provided in Dped for setting up the gunstations. So let's say you choose to use the 30 mm option, set up your dp gunstation as "30 mm" and rename your GAU8 sound file as 30CAN.WAV. Its all good for the A-10 Hog, but now every fighter in your hangar which has a 30 mm cannon will have the same rapid firing burp sound of the GAU8 Gatling. Your Mig cannons will sound this way, unless you modify their dp's to another weapon sound.
Since the A-10 is a modern warplane in an era where 30 cal. weapons are long extinct in warplanes, i knew i wouldn't need this sound option in my modern CFS2 install. So i renamed my GAU8 file as 303GUN.WAV. In modding the A-10's dp gunstation, chose the .303 gun option, but altered the ROF, range, MV and Dice to reflect the behavior and power of the 30 mm GAU8 Gatling.