Is it possible that the gas cap is first thing to check on 2003 Mitsubishi galant when service engine soon light on?

Yes, a loose or faulty gas cap is a very common cause of a "Check Engine" light in many vehicles, including a 2003 Mitsubishi Galant. The system monitors for leaks in the evaporative emission (EVAP) system, and a bad gas cap is a frequent point of failure. It's inexpensive and easy to check, so it's always a good first step before resorting to more complex and costly diagnostics.