I have some limited info but on a gas gauge from one of my Nova's is they have this external shunt resistor across the back two terminals of the gauge and if this shunt resistor is not in place or of the right value, then doing the two tests of grounding the sender wire to get the gauge to read empty and then having it connected to nothing to get the gauge to read full can give you the impression the gauge is fine but you have to do further tests with resistors on the sender terminal of the gauge to really confirm things.
This is a gauge without the shunt in place on the back of the gauge and then has a 33 ohm resistance attached to the sender wire of the gauge.
It does not read correctly as it should read like below with the correct shunt on the back of the gauge.
Now here is the same gauge without the shunt in place on the back of the gauge and then has a 75 ohm resistance attached to the sender wire of the gauge.
With this 75 ohm resistor the gauge does not read right but should read like below with this 75 ohm resistance AND the gauges shunt in place.
The more I talk about and research things about the fuel gauge, the more I feel one needs to do more than the two tests with grounding and then not grounding the sender wire. One needs to buy some resistors such as one for testing a 0-90 ohm system with getting a 22.5 ohm resistor, a 45 ohm resistor and a 90 ohm resistor. By then putting in a 22.5 ohm resistor between the gauges sender terminal and ground the gauge should read 1/4 full. If then the 22.5 is changed to a 45 ohm resistor then the gauge should read 1/2 full. If you then series the 22.5 ohm resistor to the 45 ohm resistor and hook it up to the gauges sender terminal and to ground then the gauge sees 67.5 ohms of resistance and then should have the gauge reading 3/4 full. Then as a further test, putting in the 90 ohm resistor by itself should make the gauge read Full. The same kind of testing can be done with the older 0-30 ohm system and checking things with a 7.5 ohm resistor, a 15 ohm resistor and then a 30 ohm resistor.
Keep in mind some replacement gauges have the gauges shunt resistor now built inside the gauges housing while an original had it externally bolted in place and if one did not know what was going on, bolting on an external shunt to a gauge that already has this shunt built inside would throw off how well the gauge would actually read.