Is a good GRE score evidence of merit?
Short answer: No, not at all.

Long answer:


  What I understood from the official GRE manual:


GRE, just like any other multiple choice exam, requires fairly specific preparation. You need to understand how to go about analyzing their standardized types of problems, methods of elimination of the answer choices, and even sometimes think like the person who's writing the question. Getting a good score means that you've acquired these skills and can use them rather efficiently in the 3 hours you took the exam. This is what I understood by reading their official manual.


  My personal opinion:


Doing well on GRE does probably also mean that you can, in fact, learn how to do other things, given that they are presented to you in "standard packaging". But so does an undergraduate degree where you get to take classes and prove by taking exams that you can efficiently use what you learned in those classes.

However, none of these (and no other single thing) actually "guarantees" that you will do well in graduate school, regardless of what program you go into. It's just a metric that you should be able to do well, if you are passionate enough to do well in graduate school.