I had more-or-less the same question a few months ago while I was doing extensive harness modifications (upgrading to an internally regulated alternator). In my case, the fuse by the battery had been cut out of the harness. I didn't quite understand what that circuit was doing until VetteVet set me straight.
You should be seeing power on both wires and the gauge is an ammeter. The ammeter circuit starts at the red wire junction downstream of the alternator, through the first fuse, then to the gauge, back out of the gauge, through the second fuse by the battery, then ends at the junction block by the battery. It parallels the main red wire from the alternator to the battery, thus letting it measure system load. VetteVet included a helpful
copy/paste in another thread that helped me see what that circuit was doing.