What would make my voltage gauge read low when battery is good

it is common problem on early 2000 chevys on instrument cluster / and or oil pressure sensor that make gauges go haywire ..... check this first before you panic thinking you have major prob .............

check your starter if low voltage .or the bearings in your motor could be going out causing high locked rotor amperage whenever on Startup that's why you can't get enough voltage to turn your motor over. A sign would be long time if out oil change. Or over heated. Try to replace the starter change oil. And check oil pump.

Check to see if you have an external voltage regulator and it's probably bad mine was located on the neg vat terminal its called a generator voltage regulator in GM terms it was my problem