gas gauge goes from full to empty to 3/4 to 1/4 to empty?

Hello,

It sounds like an issue with the fuel level sending unit.
That sending unit is a part of the fuel pump assembly.
The sending unit consists of a resistor and a float.
The float goes up and down with the fuel level and connects with different values on the resistor panel.
Over time the resistor panel can go bad and that will give you incorrect fuel level. It can read higher or lower than what is in the tank.

In order to replace the level sending unit you will have to take out the pump from the fuel tank.

https://www.2carpros.com/articles/how-to-replace-an-electric-fuel-pump

I have attached diagrams for your reference.

Please let me know of any questions.
Thank you.