OOOOh...that may be your problem if you didnt buy a sender matched to the gauge. A temperature gage is really just a volt meter and the resistance calibration of the sender is matched to the full scale reading of the gauge.
In fact, there are several factory senders for Chevy with different resistances....check with your gauge manufacturer and they should be able to tell you the correct resistance.

Also, the grounding on a one-wire sender is thru the threads to the intake....dont use teflon tape or too much pipe goo and increase the resistance