I see you cut off the connector which is keyed to insure connection is made correctly. Did you see my mention of how it's wired in case you mixed it up and blew the gauge.
You don't usually test a gauge for 12V across the terminals. You test the 12V wire to see that it has 12V at it's gauge end while it's disconnected from gauge and key on. You test the other wire, the sender wire, with it disconnected at both ends by doing a continuity test to be sure it is viable all the way from it's tank end to it's gauge end. Once that is done, you can start testing to see weather it's a bad gauge, bad sender or ground.
As far as ground, you said the sender flange tab has a wire to the cab, and is good. The gauge is grounded, I believe, by a lug on the back of gauge that kisses up against the cluster metal housing, housing mounted to dash, and so on, back to the battery thru the body. You can put a alligator clip wire on that housing and run it to the battery to be sure it's grounded.
These are the recommended tests. Two different methods, same tests. Report what you find out.
Method 1: Key on
Take off wire from the sender, the gauge should read FULL....or even way past full. If you lose ground it will go to FULL also.
Then ground that same that same wire to a good ground. The gauge should read EMPTY.
Method 2: Key on
This is from "The Filling Station" web site
Remove the wire from the contact stud on the sending unit and ground it to the chassis. If the gauge now reads Empty, the trouble is either poor grounding of the sender or a bad sending unit.
If the gauge still reads Full, touch a grounded test wire to the sending unit terminal on the back of the dash gauge. This terminal is often marked with a red tag. If the gauge now reads Empty, the wire from the dash to the sending unit is broken or the connection to the dash gauge is corroded.
