This is not an easy fix as the level sensor is just a sensor that commands the pcm on what voltage to display for the level in the tank. Here is a schematic of the level sensor.
As far as the gauge goes, as I said it works directly off of the pcm. It is adjusted by a class 2 signal from the pcm. This type of message cannot be tested with a voltmeter. It must be checked with a class 2 scan tool. Let me know if you have any questions, I think I understand what you are trying to test, but its not possible with the way this is wired.