oldkoot,
In a round-about way, your post above proves the gauge works. Although disconnecting the sending unit wire from the gauge and grounding it wasn't necessary, it did show that the gauge swung to FULL with no wire attached...this is good. Then, if I read it correctly, you attached a ground wire to the sending unit post on the gauge and with power applied it read EMPTY..this is also good. According to Hoyle, this means the gauge is working properly.
One would then assume that there is a problem with the wire to the sending unit, the continuity to ground of the sending unit, or the operation of the sending unit itself.
You mentioned your truck has a 12 volt system and that the gauge is a aftermarket 12 volt gauge. This means no voltage reducer before the gauge..right! The gauge and sending unit were purchased from one of the specialty suppliers as a set made specifically for your truck and intended to work together..right! Don't give up, I can smell success.
Stuart
