If you are talking about a stock AD truck, your gas gauge is a small ammeter. What that means is that the sender in your tank is a varaible resistor that determines how much current flow there will be and thus how much your gauge will register. If you jumper the tank sender to ground at the tank, your gauge should read full. The gauge wiring is not fused or protected in any fashion so it is not a fuse problem or circuit breaker problem.
