I think his method, if something so obvious (which is not to say easy) can be called a method, is to discuss the issue and interact with the person long enough to build up a model of the person, what he values and fears most, and then probe for weaknesses & biases where that individual seems most susceptible, and follow those weaknesses—again and again.
If so, the method is sloppy. The descriptions I have read of the pre-conditions for Gatekeeper participation have a giant hole in them; Eliezer assumed a false equivalence when he wrote them.
Caledonian2 comments on Einstein’s Superpowers