Gauges used on gas systems are 50-100mm in diameter full safety pattern. The smaller sizes, usually fitted to regulators, may have either NPT ( taper ) or BSP ( parallel ) threads and they are NOT interchangeable. BSP thread usually have a locating spud on the face on which an O ring or seal is fitted.
High pressure gases are extremely dangerous. If you are in any doubt, STOP and ask a competent Gas technician.
Contact Alarm gauges can have one or two contacts and are usually designed to “break” on falling pressure i.e “break to alarm”. Usually, alarm signals MUST be run through an Intrinsically Safe Relay if being connected to a flammable gas manifolds or a gauge/ manifold in an ATEX area.