My temperature gauge hasn't been working for several months.

Best to use a scanner to see the actual sensor reading and then compare that with the gauge reading, in order to isolate where the problem is. I do NOT mean a parts store mini-scan, I mean a real Pro level scan tool that can read the data input-output from you engine and instrument panel, not just codes. I recommend going to a shop with Technicians who know this stuff, your mechanic is either lazy or does not know, neither of which is a good situation. here are some shops: https://repairpal.com/chevrolet-repair-in-arcadia-california

FIRE YOUR MECHANNIC FIRST SECOND...WHEN DOES YOUR TEMP COME ON? WHEN DRIVING FOR LONG TIME>?,WHEN YOU FIRST DRIVE IT? OR TRY USING A TOP RATE SCANNER..THAT WILL SOLVE ALL YOUR PROBLEMS.