replace battery and alternator and gage still drops
Under what circumstances and how much does it drop?
I presume that you are talking about the ammeter is showing no charge.
When you replaced the alternator, did you disconnect the battery first.
There is a heavy wire that is secured to the back of the alternator with a washer and nut. This cable carries the charge from the alternator back to the battery. If you shorted out that wire, you burnt thru the fusible link.
This is the positive battery cable for your vehicle.
If you look to the right of the positive terminal, you’ll see a thing called an amp clamp.
An electrical connector plugs into the amp clamp and informs the computer the electrical demands of the vehicle.
From this information, the computer makes adjustments to charging system for the electrical demands of the vehicle.
If there are no high electrical demands, the gauge will drop.
This ain’t your 72 Impala.