NPT used in U.S Pressure Gauges
It's pipe thread - i.e. tapered, NPTM, unless you have an oddball gauge. Size depends on the gauge.
How do you tell? Measure it.
No idea what you have for an adaptor internal thread.
How have you &quot;tightened&quot; it - Hand tight? Using wrenches? Clearly not using pipe dope or teflon tape, one or the other of which is required to actually get a seal.
If you refer to the Wikipedia page linked, you'll note that 1/8 and 1/4 (likely typical sizes) have about 3 turns difference between &quot;hand tight&quot; and &quot;effective thread length.&quot;