That's the cap. It is likely a 1/4" flare just like older R12 AC systems. For various cars I use a AC charging hose and some fittings to attach to a regular gauge. The car side of the hose needs to have a valve depressor as it uses a tire valve core looking device.

In a real pinch the valve core can be removed. What ever you do, be sure to wrap a rag around the fitting when you take things apart, systems hold pressure even with the engine off to prevent fuel boiling, high pressure fuel is a real fire hazard.