Sometimes it seems almost impossible to remove the old guage. Either the old gas has glued the guage in place or a former “restorer” has smeared some sort of goo on everything to try to stop the leak.
Here is the way I have been doing it for 20 years..works every time! To keep the wrench from slipping off the outter nut I bolt a short piece of bar stock across the wrench and tap the wrench with a small brass hammer. As the nut screws out, loosen the bar as required. My gadget is not pretty but it has worked many times and I just never got around to making a nicer one. A little slickum on the bar helps.

Once things are apart, work on the small nut with the guage in the vice. Assemble the inner nut in the vice during the rebuild.

To remove the old cork, just slice off with a razor knife….You were not planning to save it were you?

Before sliding on the new sink proof float, cut a notch so it will slide on past the bend in the guage wire. This will permit the float to go on far enough to “upset” the end of the wire after you install the brass washer.

Clean all the threads and parts and try the big nut in the tank before reinstalling the guage. Once the big nut screws in all the way, real easy , you are ready to proceed. I like to use the neoprene impregnated cork gaskets. I use no gasket goop of any kind, just coat all the gaskets and threads with a film of white lube.

Never give up and don't say can't.
Ron