What is the thread size on the back of oil pressure gauge??

Very common,in this exact application and air lines as well,example is the range shifter in highway trucks.
Air shocks also, but different manufacturers use different threads.

From a -6 to 1/8 line then to gauge is the only way, especially if you need a swivel fitting.
^^^^^^^^--------------NOT!
You could go the many yrs. old "copper" oil gauge line kit. Pobably sold at every speed shop in the world.