Hello, I have an 87 GMC Chevy Vandura. The temperature gauge hasn't worked since I got it. I had an incident the other day and it started to over heat and had I not been paying attention I probably wouldn't be bothering with this question :-) I wouldn't think the thermostat has anything to do with it. Any suggestions appreciated. Thank you

Thermostat could indeed have something to do if it is stuck closed. Checked fluid level?

Check the radiator for passages that may be partially blocked by phosphate scale (minerals), due to using tap water. You can check the radiator with an IR thermometer. I wouldn't be surprised if you had to replace the radiator and refill it with a 50/50 mixture of antifreeze and distilled water.