I'm not aware of how the AMA acts as a gatekeeper to the medical profession - what did you have in mind?

In the broad sense that they are an influential industry group that has taken a number of political positions that impact laws. Chiropractors and osteopaths have in particular felt discrediting hostility from them, as I read. Now to be clear, I for one do not personally hold either of those two fields in as much esteem as much of the American public might, so I don't meant to be partisan, but to simply say that the AMA has used it's influence to maintain the standards of what should or should not be licensed medical practice. They are free to maintain their own standards, which I do much respect.

You've made good arguments in this thread, but you lose a lot of credibility when you imply that a DO is not as good as an MD.

I guess they aren't referring to DOs with that remark, there's a field quite like chiropractic that is called "Osteopathy", in the US such practitioners wouldn't be DOs.