Lets start with the simple things, make sure there is a grounding wire from one of the securing screws of the fuel sender to a clean spot on the body.
Most common place would be a multiple ground connector by dash center bracket.
The fuel gauge sender is mounted on a rubber gasket and if the gas tank has been replaced chances are that the unit is not grounded,
Next make sure there is a 12 volt source to the gauge. Some models have a built in resistor that contains a fuse leading to the gauge.
The system works by providing variable ground to the gauge.
If you have power to gauge and proper ground then you have a defective sender or gauge.
Applying ground to gauge will make it to max out. Sender can be tested with an OHM meter or turning it upside down while grounding it and reading dash gauge ( also method of adjusting unit)
Let me know if you need additional info.