JCribb
Old Army truck mechanic
Just wanted to throw this out there, the gauge sender JCribb listed, while the correct part, may not work correctly with the gauge. The gauge may read very very low with this sensor. Hopefully the supply chain has been corrected by now but as of a couple years ago the sensor, while it is indeed the "authentic GM part", did not provide the correct values for the gauge to work properly.
I am of the same opinion on sensors and changed both the ECT and the gauge sender at the same time, using genuine AC delco "Original Equipment" parts. In my case, I found that the gauge reading was very low with the new sensor, like truck fully warmed the gauge would be at 9 o'clock. After reading up on the issue I found a lot of people had the same problem and said in this case the aftermarket part worked better.
I ended up just putting the 24 year old sensor back in. I think I need to revisit this now. I hate relying on old sensors to tell me what my car is doing.
TLDR; if you change out the head sensor and the gauge reads low, it's a problem with the new design of the sensor. Get a sensor from a different supplier other than AC-Delco.
I won't disagree with this, because my factory cluster (94) reads barely above 100 when the stock 195* thermostat opens, so there's definitely a lot of play with the gauge, my suggestion is to run an aftermarket temperature gauge along with the OEM one, as that is what I do with mine.
I used aftermarket sensors in both locations, and the gauge one never read at all, and the ECM one made my engine run rich, so I guess that one is luck of the draw? Mine were from Autozone, I did not try any other stores for their house brand.
For what its worth, I don't give the factory cluster any hope when it comes to readings, the technology in those days was sub-par at best compared to today's standards.