was green gas, from a company called Toy Jack.
People started using the name interchangeably with "green gas" the same was as we use "Kleenex" (brand name) for facial tissues, etc.
HFC134a was also known as "Blue Gas" and was marketed by Tokyo Marui under the name "Gun Power", KWC as "Power Bomb", etc.
Outside of Japan, most guns were designed to use the more powerful Green gas.
@OP: what's the gun in question?