Here are the calibration values for the gauge:
0 PSI: 1 ohm
40PSI: 46 ohms
80 PSI: 87 ohms
I calibrate them with a decade box, which simulates the resistance of the sender. Your sender resistance readings are not too far off, so I think the problem might be the gauge itself, most likely a sticky meter movement, even though you got full scale deflection when you disconnected it from the sender.