Did you do this:
Connect +6V to the gauge power terminal. Connect the battery negative or ground lead to the gauge mounting plate. At this point, the gauge needle should go to "full" or beyond. Now connect the gauge sender terminal to ground (or the mounting plate). The needle should go to "empty" or beyond.
If you did and it's still stuck you can replace just the gasoline gauge.
I doubt that anyone would rebuild the gasoline gauge.
The master parts catalog says:
Group # 3.108 Gauge 49-53 Commercial, Utility
54-55 FFC (Flat Face Cowl)
Part # 1517169Partsvoice says the below vendors each have one NOS in stock.
Nos Parts Source
Olathe, KS 66062
913-220-5746
Modern Chevrolet Company
WINSTON-SALEM, NC 27101
(888) 306-0825 ask for parts department
Collectors Auto Oroville Gm
Blaine, WA 98230
800-414-4462
There are couple on eBay also, punch in the part number.
Go
HERE for 2nd option and pics
What color is your needle?