[Accelo]

The gauge actually reads voltage. Millivolts to be exact. A millivolt is one thousandth of a volt. It's a volt gauge that is calibrated and marked in amps. It is in what is called a shun circuit. All the connections have to be perfect for this to work. Both ends of the gauge are hot and both terminate at the same wire just in a different spot on the wire. Confusing isn't it? This is the reason gm with to a volt meter with a higher range actually marked "volts".
None of this matters. All you need to know is the terminals have to be really clean.
One side terminates at the little black connector next to the battery. Pull out the wires and clean them with sandpaper and clean the nut and stud. Cover with grease or something so the corrosion will not come back.
Check the two inline fuses. The connections for this connection have to be spotless. That is why the fuse is fully enclosed from the factory. There are two of them one in each front corner next to the fender and core support. Confirm the fuse is there and clean them till perfect. Then do not run the motor just turn on the high beams and watch the gauge. Most the time you can get them to operate with these simple fixes.