Temp Gauge reads car overheating....engine temp. is fine

2005 Pontiac Vibe wagon...over 300,000 no issues until recently. The temp gauge goes to overheat. Here is what has been done in the past two weeks. New thermostat, proper coolant, new radiator cap, sensor, and just installed a used gauge panel. Fan is plugged in....heat sensor gun shows normal temps. Help! I'm a single gal who doesn't want to be taken advantage of...any ideas? Thank you!