At least a Class I machinist would be my request for a gunsmithing school to require before entrance or upon graduation.
Yes, for sure there are excellent gunsmiths that are not certified machinists and who can build very good rifles because, through experience, they have learned how to use the "tools of the trade". They also know how to build and fit stocks, mostly wood stocks and fiberglass, something that goes beyond a machinists trade. These are people who have a deep tribal knowledge but I think there are not a lot of them with this deep knowledge. I've seen enough botched "gunsmith" jobs to know that.
When people search for a "good gunsmith" they have only references and anecdotal tales to guide them. We need a rating system and national standards for gunsmithing.
A good gunsmith should be able to do many things beyond proper chambering, threading barrels (either end) and bedding actions into stocks. They really should be able to take a barrel blank and machine flutes or make it octagonal so that the top flat is exactly at 12 o'clock, fit front and rear sights into dovetails they machined, and properly fit bottom metal so there are never any feeding problems.
When they fit a scope they should know when hand lapping scope rings is warranted, how to level a scope and how to fit fore and aft it to their client. They should know when a zero, 10 or 20 MOA rail (or ring mount) is justified and be able to explain it to their clients. Generally, as a professional courtesy, a scoped rifle should not leave their shop without being bore sighted.
Good gunsmiths should be able to hand lap a barrel, make a given trigger as good as its design will permit, add forearm and pistol grip caps of wood or other material so it looks perfect, and of course know "the law" as it pertains to firearms modifications and of course, be a good businessman and employer.
That's a lot to ask of one person. If they need help in the business or employer department they should seek professionals who can help them. I think this is where a lot of truly good gunsmiths struggle.
A good gunsmith will also tell his clients when he cannot do a certain operation on a firearm either because he does not have the skill or the tools, then refer the client to one who can help them. I have had this happen on a few occasions.
I have used the services of an excellent gunsmith/machinist, Neil Jones of Sagertown, PA. His advice and work always was first class. His reloading dies are world class.
Eric B.