There really isn't a standard thing called a 'gas compressor'. A gas compressor can be any number of things. An air compressor however, is intended to compress air and it is a subset of all different types of gas compressors. The adjective "gas" put in front of "compressor" only means that the fluid is in the gas phase. You can also describe a compressor as a "flammable gas compressor" or a "toxic gas compressor" or "oxygen gas compressor". In each case, there are features in the compressor that allow it to compress that type of gas. A toxic gas compressor might be a diaphragm machine that has no dynamic seals. An oxygen gas compressor will have special materials to minimize chances of a fire. Flammable gas compressors will have a way to capture any leakage past piston rings that can be routed to either a vent stack or back to the inlet. Each type of machine is designed to a specific criteria dependent on the type of gas.