propane, so essentially green gas. With gas guns FPS is less important. It might be 330fps on a .20 bb, which is about 1 joule. But with a .32-.36 it could be 1.3 joules for example. Because an AEG has a fixed volume of air it tends to either keep the same energy or loose it with heavier BB’s (depending on barrel:cylinder volume ratio) unless it’s specifically built to joule creep.
A gbbr is different. The heavier the BB is, the longer it’s in the barrel for, which generally means the more of the energy the gas can impart on it. It’s another reason putting a longer barrel in a gas gun can make the fps go up considerably. Longer the barrel, the longer the BB is inside of it, the more energy it absorbs. Which can be advantageous or cause cycling issues if you go too long.