Jimmy,
As Paul said usecthe cork only . 1 gasket behind the glass lens in the inner nut with the metal plate and then the nut. They include 2 small cork gaskets but you only use 1 of them. Do the inner one in the bench and get it good and snug tight. Do not do “hand tight” as was suggested above, you will be sorry. I’ve done hundreds of these.
Then making sure all the ridges are clean on the tank and back of gauge use the cork setting it in the hole and then the gauge followed by brass thrust washer and outer nut.
Snug well and check Aligmment . If gauge moves adjust and retighten.
The best tool for the outer nut is the original steel 5z wrench. Most don’t have access to them but if you find one ****** it up. They are the best. I don’t use any sealant and haven’t had leaks.... that’s up to you .
FYI make sure gauge is calibrated by checking before tightening in.
Larry shepard