The float that tells your gauge how much gas you have may be stuck or there is a problem with the system that sends that information to the gauge.
Purchase an inexpensive hand pump and pump it out.
Either hungry, empty or thin would be opposites for full
The fuel gauge sending unit in the fuel tank is probably the problem.
Pump the fuel out with a hand pump. You can get it all but a gallon or so.
Sending unit is bad, or the unit is no longer grounded. Most gauges work on resistance, 0 resistance being empty and 90 or so being full. if no ground is present then the circuit won't be complete, on the other hand, if the wire is shorted the gauge should show full.
The sending unit, located in the fuel tank, is bad.
Empty hand is Karate, a Japanese martial art.
right click, right triger, or tap on it with an empty bucket in hand
The way of the empty hand
You should be able to have access to the gauge from under the car and in the front of the tank. It would be good to have a new rubber seal on hand. Check the wiring first, sounds like the gauge is grounding out. Do this on an empty tank of course.
Hand Gloves
Karate literally means "empty hand" (kara=empty, te=hand)