For the OP--get the ejector bolt tool, it is cheap insurance. Two, replace the roll pin with a new one when reinstalling--cheap insurance in getting it out again or damaging the bolt during reinstallation. Third, use a proper sized roll pin punch for removal. I remove all of my ejectors and extractors for headspace checking because I do not always purchase the bolt and barrel from the same supplier on builds. Use some Kroil or other penetrating oil too for assistance in getting the roll pin moving out.
Blue68f100 is correct above on the design of the ejector removal tool as it works from one side.
Unless you have a military supplier for the headspace gages with cutouts for the ejector, the last time that I have checked, these have vanished from the consumer market but you might find one on ebay if you look long enough or on forum wts/wtb sections. It would be a piece of cake for a machinist to alter one though. I believe a guy named Bill Ricca used to sell them though.
GI gauges
Part Number 7799734
NSN 5220-00-070-7814
https://www.nsncenter.com/NSN/5220-00-070-7814
Here is what Manson has to say about the subjects,
https://mansonreamers.com/2016/09/01/ejectorextractor-relief-on-headspace-gauges/