You first want to use the Rubber seal in your Dual Filter and run a test. (This basically safely chokes the machine and will create the most amount of pressure). if your gauge is in fact working then it might be your water valve.
If you don’t have any readings while running it with the rubber then i would initially suspect damaged or weak water pump, followed by solenoid.
Check the line that runs from solenoid to gauge if you dont see any water moving through there im pretty sure it will be the water pump. You can also check those lines for obstructions
If after doing these test you don’t have any readings then yes the gauge is probably shot. There’s videos on YouTube to replace. Not extremely hard to do, just time consuming disassembling and putting back together