Overflow always fills,,,,radiator 1/2 full,,,when driven,,,gauge

No just the coolant surge tank.

Yes it can be the cause, but a leaking head gasket or system overheat can also cause the bubbles.

Yes if stuck closed

Wait for the cooling system to cool down. Remove the radiator cap, start engine and watch for bubbles in the tank. If you have bubbles the possibilities of a bad head gasket is suspected. To verify a suspected head gasket, leave the radiator cap on and tighten. Start the engine, using a heavy towel or rag loosen the radiator cap SLOWLY. If pressure is evident only after a minute of running.....=Blown head gasket!

1. Check the radiator cap.
2. Check the reserve tank, whether water level is maintained or getting filled. If it's getting filled, there should be a leak in the radiator or gasket

Sounds to me like it might be more of a leak than repair/sealer can handle. It never hurts to try though. But if it doesn't work, you might have a little more cleanup to do when you pull the head. It may also be a cracked head as opposed to just a head gasket as well. keep thet in mind. A bad head gasket will cause an overtemp, not the other way around. As for the temp gauge, someone may have worked on it before. sounds like it's hooked up backwards. check it out. if the gauge is bad, it's probably cheaper and easier to get an aftermaket one.

The first things to do are: 1. Make sure the radiator is full when the engine is cool (check the fluid in the radiator as well as the overflow tank). 2. Make sure the thermostat is working. 3. Have the lower intake manifold gasket checked. The Grand Prix SE came with a 3.1L engine (standard) and these engines are famous for lower intake manifold gasket problems.

to much water in expansion bottle fill to level only when cold

head gasket does the overflow bootle have bubbles in it when idleing warm if does most likey rear head gasket.

is it possible it is your overflow bottle leaking or too full? If not These cars are bad about headgaskets. Start your car remove radiator cap and let it idle till it is up to temp and top radiator. If you have a leaking gasket or cracked head you would most probably see bubles and the water will try and come out of the radiator fill. What hapens on many is while your driving, combustion gases force the antifreez out into the overflow and it fills up and spills over.