Above the oil filter, there are 2 cap plugs that are 1/4" square drive (use a 1/4" ratchet w/ no socket).
That cap plug is 1/4"NPT male, so you will need to get an adapter if one didn't come w/ the gauge to go from 1/4"NPT male to 1/8"NPT female. Just hit up your local hardware store in the plumbing aisle.