I have 1999 Bonneville sse whose gas gauge does not work correctly ,it will show the correct amount on start up but when driving or after awhile it will show incorrect amounts from full to totally empty even after I fill up...here is the question how do you change the gauge? and is it connected to the fuel-pump assembly?
do I need to change the fuel-pump also?

The level sensor is an integral part of the fuel pump sender. You'd need a new sender from the sound of it.

This is a 97 assembly, but should be about the same look at yours. The yellowish float is attached to the resistor part of the gauge via a metal rod. The little fingers that ride the resistor board tend to break off over time.

Ok that answer one of my question very well , do I have to drop the tank to take the fuel-pump assembly out or is it in the trunk where I can take it out?

Gotta drop the tank.

I always suggest picking up a set of straps. They are cheap and good to have on hand.

Now does that include my '97 GTP? I noticed a hole in the trunk that looks like a fuel pump access. I have to get in there and fix mine too... reads full all of the time.

No, W bodies were redesigned for 1997. They have the super duper fuel pump door that can be accessed from the trunk if you pull the carpet back. H body guys before 2000 hate you.

ps.. you W body jerks also have a "kit" that'* cheap to replace your fuel gauge sender that doesn't require a whole pump assembly to be replaced.

Awesome !

You could've swapped that while waiting for Rad.

LOL! If I had the money for a new sender, i woulda called a tow truck!