1997 Chevy Camaro

When you replaced the thermostat, did you bleed your system of air? This is a necessary procedure on your LT-1 motor because it's a reverse flow cooling system.

When your car is cold, open the cap for the surge tank. Then start your motor and place your heater controls on full hot.

Watch for the coolant level in the surge tank to drop suddenly. When it does, rev the motor up to at least 1,500 r.P.M.S and squeeze the upper radiator hose several times until you see no more air coming out of your system.

As for your temp gauge:
The only reason it would "take a long time to move", is either it's stuck open and not operating, or it's the wrong thermostat for your vehicle and it's opening too early. (Not slowing down the coolant like it should)

Coolant needs to flow slow enough to be able to pick up the heat from your motor, and to redeposit it in the radiator.

Yours may not be doing that.