gas is a state of matter. In the context of firearms, it refers to the chemical result of burning propellant (gun powder).
The chemical reaction of burning propellant transforms it from a solid to a gas. The natural volume of the gas is several hundred times more than the propellant in its solid state, which creates a state of very high pressure (tens of thousands of PSI). The pressurized gas exerts force against the base of the bullet, resulting in rapid acceleration.