Gas gauge reads full one minute then reads empty?

Hello,

It sounds like an issue with the fuel level sending unit.
That sending unit is a part of the fuel pump assembly. It is consists of a float and a connector with different resistance value. As the fuel goes up and down on the tank the float moves with it and connects to different resistance reading, which translates into the fuel level gauge.
The resistor can go bad overtime and will trigger incorrect reading of the fuel on the gauge.
In order to replace the fuel level sending unit you will have to remove the fuel pump from the 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.