thermostat may be sticking and that cause the gauge to show hot, have it checked soon, you could be headed for bigger problems
by the lower intake manifold while you are doing this you should replace the intake manifold gasket anyway because it will leak sooner or later.
Get a Bigger Radiator
it could possibly have either a blown head gasket or intake manifold gaskets leaking
Probably a gasket leak. Could be the intake gaskets or the thermostat gasket.
I was having a problem with my intake manifold gasket on my 2003 Pontiac grand am and i took it to a shop it was 600 dollars parts and labor you can get it much cheaper i just chose a mechanic i could trust. now it may not be intake manifold gasket the best way to tell is to lift your oil cap look at the backside of it and if there is a blueish tint liquid on the cap then your getting condensation which means bad intake gasket. But it can also be your thermostat housing messing up which would be much cheaper.
3.1 engine leaks at the intake gasket
I have a 2000 Pontiac and my was doing the same thing. It turned out to be my heads were warped. You can smell it coming out of your tail pipe if it is your heads.
The thermostat is located near the intake on a 1999 Pontiac Grand Am GT. The thermostat can be located by following the upper radiator hose directly to the thermostat.
Can be many things, but considering you just changed the intake gasket, I would suspect you did it incorrectly or have knocked a vacuum hose loose somewhere. You have a vacuum leak somewhere.
The 1999 Pontiac cylinder head gasket bolt torque specification is 160 pounds. The intake manifold bolt torque specification is 90 pounds.
how to replace a distributor gasket on a 2000 Pontiac grand am 3.4l
Slightly messy this way... Make sure truck is cold. Unbolt thermostat housing located on the top front of the motor, with the upper radiator hose attached to it. remove housing, remove thermostat. Clean gasket surfaces on intake manifold & thermostat housing. Spread gasket silicone on intake thermostat flange. Install new thermostat, install new gasket on top. Spread gasket silicone on underside of thermostat housing. Bolt housing on intake. Remove radiator cap & top off with 50/50 coolant/water.