GM-GMC: temperature sending unit, temperature gage, temperature sending unit

Question
My temperature gage is reading high I have replace
the thermostat and still get same results my
next guess is temperature gauge
On a 1990 silverado where is the temperature
sending unit located

Answer
You did not specify an engine, and I have not worked on a vehicle like yours, so I cannot give more than a general answer.

GM usually installs the temperature sender in the cylinder head, often next to one of the spark plugs.  It is usually a brass hex with a single wire attached to it.