What is an alternator gauge?

This is a misnomer. Electrical system function is gauged by either a voltmeter or an ammeter. The voltmeter measures system voltage, and should read around 18 volts with the engine running, 12 volts with the engine off, and 8 volts when starting. An ammeter, less common, measures current flow between the battery and the alternator. It should read a positive 2 or 3 amps during normal operation, indicating a constant charge to the battery. Just after starting, it should read a higher positive as the alternator replaces the current drained by the starter. If it reads negative, the alternator is not making enough current to meet the car's needs and keep the battery charged. This may be caused by too many accessories (lights, stereo, video,) a slipping belt, or a malfunction of the alternator itself.