I haven't tried CO2 on my Gauntlet .25, but I have used it on a similar gun in .25 caliber, but with a shorter 18..5" barrel. JSB .25s ran 650fps around 70 degrees.
A word of caution... make sure you have spare o-rings on hand prior to charging with CO2. It may take months or it may take as little as days, but you will need them.