A gas fire is not a plasma because it does not meet the conditions required for plasma formation. A plasma is a state of matter where the gas has been ionized, which means that some or all of the atoms in the gas have lost or gained electrons, resulting in positively charged ions and negatively charged electrons coexisting in the gas.
In contrast, a gas fire is typically fueled by the combustion of a hydrocarbon gas (such as methane, propane, or butane) with oxygen in the air. The combustion process produces a flame that emits heat and light, but the gas molecules in the flame are not ionized, so it is not a plasma.