Talked to George Gardner to see what he can recommend.
A gas gun requires different powders to keep things from breaking. In general, they prefer faster powders to provide a good pressure "pulse" and short duration to keep things from breaking. However, George may have designed a bleed off system or something else that would allow any suitable powder to be used.
IF, you find out you can use any suitable powder in your gas gun, I have had good luck with IMR 4064 and RL-19 with 120 gr. class bullets, and either H 4350 EXT or H 4831SC with 140 gr. class bullets in both of my bolt guns.