Yes, GAS is a Scrabble word!

Letter values in Scrabble US: G 2, A 1, S 1.

Is it a valid word?

GAS is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

the state of matter distinguished from the solid and liquid states by: relatively low density and viscosity; relatively great expansion and contraction with changes in pressure and temperature; the ability to diffuse readily; and the spontaneous tendency to become distributed uniformly throughout any container
a fluid in the gaseous state having neither independent shape nor volume and being able to expand indefinitely
a volatile flammable mixture of hydrocarbons (hexane and heptane and octane etc.) derived from petroleum; used mainly as a fuel in internal-combustion engines
a state of excessive gas in the alimentary canal
a pedal that controls the throttle valve
a fossil fuel in the gaseous state; used for cooking and heating homes

Also: gaseous state, accelerator pedal, gas pedal, natural gas, shoot a line.