As others stated, really need to see exactly what you have.
Sounds like a standard pressure gauge, not an SPG. All you need is a leak. Are there pipe threads involved? Try loosening them.
My choice (if it is how I am thinking it is, but no way to really know as I am guessing like the others) is to partially disassemble the gauge. Get to the brass tube that runs the gauge linkage. Start nipping away at that with a pair of big wire cutters. Tank valve closed. Not a flat out cut through it, at least right away. You want to make small nips until you get a tiny leak. Wait, a little bigger nip, eventually you will have destroyed the gauge. Then take it off.
You will NOT be able to remove the yoke adapter of the DIN connection while under pressure. There is a LOT of surface area. At full tank pressures, you are asking to slide metal on metal with the weight of half a car on it. Threads just lock on under that pressure. A small diameter pipe thread still has the pressure, but not the surface area. So there is a chance that will unscrew under load.
Expect something to be damaged. plan the damage to something that isn't that bad to replace. And it can be damaged safely.
