Beings how the gun just swallowed a no-go gauge, how do you know how much more it might could still swallow?
Excellent question/observation! The answer is: That's what the Field Reject gauge/specification is for.
Again, a No-Go gauge does NOT (necessarily) mean a rifle is unsafe to shoot. It just means that headspace is unacceptable for a new rifle or barrel.
It also suggests (as does your question) that further investigation be done to find out how much
excessive the headspace is.