Temperture gauge is pegged on high

[gmc684x4]

Is the sender for your model of truck ?
Example 96 99 vortec 350 engines use a different range temp sender then the 67 72 trucks

To make one work you would need to install a 67 72 temp sender in intake or thermostat housing

To use in stock location one needs to have the threads cut down as a vortec head has smaller port for temp sender

Your guage is proubly fine i think you have the wrong range of sender?

Sender range and guage have to match or they will not display correctly

What motor transplant did you install?