At least take off the bottle. But you won't need to tighten too much. Go easy with it, and dont sweat that the gauge isnt lined up pretty when your done. I spent a l in NY time trying to line it up pretty, and tight. Major waste of time
These stupid gauges are poop. Few are accurate and they all insist on using the BSPP thread form. I hate BSP threads. Only the Brits use it. They use a 55 degree thread form like Whitworth threads rather than the modern standard 60 degree for metric and US national thread forms and specifically, the BSPP standard seals at the base not the thread itself. This needs a sealing flat washer at the bottom of the hole and the finish of the hole bottom must be perfectly smooth and scratch free as well as the gauge thread base in order to seal. This washer can be nylon or fiber, not rubber. I prefer fiber as it doesn't flow under pressure and they seem to seal better at these high pressures. If you want to inspect the sealing surfaces of the gauge hole or inspect/change the sealing washer, you will have to drain the tank of course.
I'd be careful. I twisted a gauge off on my Wildcat while trying to install in the bleed port. "Before" I knew about the delrin washer. I was using a Huma gauge, and the base where the threads are is brass, and rather thin, because of the hole in the middle. Took very little effort, just twisted off. Had to use an easy out to get the broke piece out. If your doing this under pressure, and it snaps, you might get a face full of gauge.
I used a bit of loctite 545 when I put on my new gauges and didnt have a problem with leaking. There was some thread compound on my factory gauge(Im pretty sure at least) so I put some on just to be safe. If you still have a problem after removing the bottle and tightening the gauge I would try the loctite and follow the directions closely.