I would suggest to go by elimination of problem possibility.
First, check resistance between gas tank and a well know ground.
Check resistance between gauge - and a well know ground.
Check resistance between gauge - and gas tank
Check continuity of the wire between gauge and sender (disconnected).
Check wire for short to ground (disconnected)
Check if it's the same wire from tank to gauge
Do you read the same at the sender (wire disconnected), at the end of the wire Gage end (connected at the sender, disconnected at the gauge).
Check resistance between a well know positive and + at the gauge.
Maybe remove sender from the tank and with it properly grounded, check if resistance change.
If all check good, than maybe your gauge is faulty or do not match your sending unit. Double check the instructions and don't assume because wires or components are new that they are in good working condition.
J.F.
