I would do all of the above metioned work to it. If you look REALLY close where the lower intake and the heads meet if you see black plastic there then those are original LIM gaskets and need to be done. Then a coolant flush and refill with conventional green. No need for Dexcool. New thermostat and gasket, housing should be fine.

Inaccurate fuel gauge is likely the sending unit which is part of the pump assembly. Lucky for you GM made an access door in the trunk to get to the fuel pump. If you pull the rear seats out and then pull the trunk carpet back you should find a door with a few bolts holding it down. Under that is your fuel pump.

And yes, some can have the autolamp feature. My 2000 GT doesn't have it but my buddies 01 GT does. Personally I like not having it.