Once the gauge is out of the tank.....
This is important - the internal glass pushes down on the spring collar. You must loosen the screw at the top of the gauge before removing the spring collar. You don't have to remove it, but rather just loosen it a few turns.
Replace all the old gaskets with new and reassemble.
Don't forget to install the large copper gasket between the gauge and the tank during re-installation. Good luck! Email with questions.