Gauge on a shotgun that has not

If it's the 3000psi fill PBBA air shotgun-457 that only gets 3 shots so a manometer wouldn't make much sense just fill it then if you can just count to three then you refill.

When in doubt just fill to 3000psi then shoot three shots and refill or leave it then only refill when ready to shoot another 3 more shots.

https://youtu.be/Pceh4SzYhiQ
 
Knock on wood, but I haven't noticed any leak concerns on mine. It's a fun gun though.

Here's a potential idea though. When the pressure in your line meets the pressure in your gun on the way up, you hear that click in the fill port opening. You could fill to 3000 psi. and disconnect. If you grab your gun a week later, hook up to the fill fitting, and you hear that click at 2500, then you would know it has leaked down between sessions.