Railroads have the idea of the lowest tunnel on a track.<p>Rock climbing has the idea of the "crux" of a route.<p>Chemistry has the idea of the "rate limiting step" of a reaction.<p>So if an endeavour has a whole chain of things that need to go right (things that need to be mastered) for success, say A-B-C-D-E-F, and D is the most difficult thing, and there's some simple test T that predicts performance on D even for people who don't yet know A, then I'd say "good gatekeeping" would be to encourage people who do well on T to try to get to F, and advise people who do poorly on T that they might wish to spend their time and energy attempting something else, instead of putting in all the effort for A, B, and C only to never make it to F.