Alternator issue, gauge moving down after vehicle is running for fifty or more miles

I have the vehicle listed above Envoy/Denali. I am on the third alternator,
two rebuilt and a brand new one since February of this year, plus a brand
new battery. Here is what is going on: I thought the two rebuilt alternators
were defective, so, they were replaced. When the second one allegedly
failed, I told AutoZone I wanted a brand new one. I had it installed last
week and, it was fine for a few days and now, after I drove about 60 miles
straight, the needle dropped from just above the halfway mark, to between
zero and half way, yet, the battery is brand new. The connections are fine.
Does anyone know what could be causing this to happen? On the second
rebuilt alternator, the five months old battery was damaged, so, I had to
replace that, too, last weekend. The GMC dealer says I need to buy only
a GMC alternator, which is garbage, because they want to charge me over
$500.00 just to have one installed. Something is causing the alternator to
charge erratically, causing the needle to vary, and I don't want to be
stranded.