I'm new to Johnsons as well. I just finished cleaning and detailing my first and I have developed a real appreciation for the quality and workmanship.
I checked the head space by taking the barrel off and the bolt out of the rifle. Unlike Garands and M14's, the bolt does not lock to the receiver, it locks into the barrel chamber.
Insert the bolt in the barrel to see how it fits. When the teeth are aligned correctly, you can insert the bolt and turn it.
Then insert your "GO" gauge and do the same. It should insert and turn to lock the barrel and bolt together.
If the "Go" passes, then insert the "NOGO". The bolt should not insert far enough to allow it to turn and lock.
Both of my barrels closed on the "GO" and neither closed on the "NOGO" so I'm feeling pretty good.
If you do it with the barrel mounted, you will have to remove the extractor and use a punch to operate the bolt.