This has nothing to do with gatekeeping. I agree that the current publication and incentive system is broken, but it's completely unrelated to the question if outsiders are being published. The reason why you see very little work from outsiders is because research is difficult. It typically requires years of full time dedicated work, you can't just do it on the side. Moreover, you need to first study and understand the field to identify the gaps. If you try to identify gaps on your own, you are highly likely to go off into a direction which is completely irrelevant.

BTW I can tell you the the vast majority of researchers are not "enthralled" by the system, but highly critical. They simply don't have a choice but to work with it.