IIRC, the pink wire should have 9 VDC with the ignition on. The black wire is chassis ground. With the power on, if you disconnect the pink wire at the tank, the gauge should read full. If you ground it the gage should read empty. If it doesn't, the wiring, the guge panel orinted circuit or the gauge is bad. If if the guage & wiring check out, check the sender. If the sender is the correct GM 0-90 ohm unit, you should read 84-88 ohms tank full, somewhere around 40 ohms 1/2 full & 0-2 ohms tank empty at the sender pink wire connector with the pink wire off the connector. I'm betting your pink wire or where it connects to the printed circuit is bad.
Tommy