So I have a low temperature on my gauge been running 180 thermostat for 20+ yrs it has always sat at 1/4 on the gauge Pictures below so about six months ago out of the blue the gauge started reading low .
* New radiator cap
* I Put a new 180 thermostat in
* I Put a new temperature sensor in
* I tested the gauge wire to the sensor with a jumper wire to ground pegs to the hot meaning the gauge is working
* i’ve checked all the grounds
* checked fan clutch
* i’ve checked the temperature with the gun It’s at 180 thermostat housing
I don’t know what else it would be
FYI the N in the Picture = normal temperature reading

 

Usually best to change one variable at a time. Did you do that? Maybe the indicator (gauge) is "working", but inaccurately? I would expect that "normal" 180 would indicate higher. My '89 22RE is still on the original thermostat and has always indicated just a pointer-width or so below center when its colder outside and the thermosat is regulating. Just thoughts...

 

Line loss of signal to the gauge from the sending unit from old age.

The thermal cycling after 20 years does a number on the wiring harness

Then on those stock gauges not much better then a idiot light a needle movement of a 1/4 of the gauge might only be 10 degrees depending on the supplier

 

Yeah, the gauge on these trucks is generally a pretty inaccurate device. Especially as they get older and older. As long as the temperature read with the gun is 180 on the thermostat housing, I would just say the gauge is getting older, and call it even. I would just bear in mind the "new normal".
Alternatively, if it's really bothering you that much, I see a couple possibilities. First, are you sure you replaced the right sensor? IIRC, there are two, one sends temperature info to the ECU, the other goes to the gauge. Did you replace the one for the gauge, not the ECU?
You can check the wiring with a jumper from the sensor to the gauge to see if/how it differs from the stock wiring.
Finally, you can replace the gauge itself. I did it in my pickup, chasing a high temperature condition it was showing. But that was back around 90 someplace Still, it's a very easy task, not counting getting the gauge cluster out of the dash panel. Two nuts, it falls right out. Impossible to put it wrong, as long as the bolts on it go in the right way up.

Finally, there is a 190° thermostat available for our trucks. It's what I use. The hotter it runs, the more efficiently it burns the gas. Within reason, naturally. With a 190° thermostat in them, my pickup INDICATES just above 1/2 on the gauge, and my 87 4Runner indicates just above 1/4, with the heater off. With the heater on, on a cold day, it indicates just below 1/2. I think those examples just go to show the accuracy, or lack thereof, of the gauge systems on these trucks. As long as the gauge stays below the upper white line, I don't worry about it. The only time one of them DID get up to there was the 4Runner blew the top off the all metal 3 core I was using in it, from back in Yuma. Nasty, miserable, HOT place, Yuma.

I wish you all the best with your situation...
Pat☺