Testing off-vehicle is a very bad idea. No bench tester can run a generator like it is run by an engine, and none of the vehicle's wiring is included in the test. AC generators can easily require over five horsepower to run wide open. In-store bench testers rarely even have a one horsepower motor.
Also, do not rely on a dash gauge for accurate voltage readings. Those are only good for observing when an abnormal condition is occurring. You can start the testing by measuring the battery's voltage, while the engine is running, with an inexpensive digital voltmeter. It must be between 13.75 and 14.75 volts. Your observation the dash gauge was dropping suggests you are going to see that too with the digital voltmeter, and that points to a failing voltage regulator. That will not show up on a bench tester if it takes a few minutes to occur. Tests on the off-car testers are done and over in a few seconds.
If the charging voltage stays within the acceptable range, that means it is okay to do the second part of the tests, but that requires a professional load tester. Specifically, it is "full-load output current" and "ripple voltage" we want to see. Since the 1987 model year, GM has had a real big problem with their redesigned generators that develop huge voltage spikes. Those can damage the internal diodes and voltage regulator, and interfere with computer sensor signals. When one of the six diodes is defective, the most output current you'll be able to get is exactly one third of the rated current, and ripple voltage will be very high. 30 amps from the common 90-amp generator is not sufficient to run the entire electrical system under all conditions. The battery will have to make up the difference until it slowly runs down over days or weeks.
The battery is the key component in absorbing and damping the harmful voltage spikes, but it loses its ability to do that as it ages and the lead flakes off the plates. To reduce the high number of repeat failures, always replace the battery, unless it is less than about two years old, any time the generator needs to be replaced.
The place to start now is with the professional on-car charging system test. If the problem only occurs after the engine has been run a while, have the tests done at that time, not when the generator is still cold.
Wednesday, January 18th, 2017 AT 5:41 PM