my trucks volt gauge drops and battery light comes on while im driving

What is your question?

I've already replaced the alternator, battery,belt and cleaned all grounds. when i pull over and turn the truck off after a few seconds i see the fuel gauge jump up to half then i open the door and it drops. when i start the truck back up everything works normal and i can drive a little more than it happens again.

How long has your 2002 GMC Sierra 1500 had this problem?

a few weeks