If a student acquires a good score in GRE, does it guarantees that he has a good merit to earn a good grade in a taught graduate program?
Short answer: No, not at all.
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.
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.
My answer would be depending on which GRE are you referring to?
If it is a specialized GRE, like the one of Computer Science, I do agree that it shows some merit, since it test basic knowledge the students should have in order to start doing useful things as soon as they start their PhD.
If you are talking about the general GRE, I think it does not reflect well in some abilities like problem solving and scientific comprehension, since its math part evaluates how fast can you make calculations, not how deep is your knowledge of basic math.
Again, there are a plethora of factors that will affect whether you get in or not in a Grad Course, as far as I remember GPA also affects in some extent, since top universities want people that were at least in the top 1%
The problem I see with the GREs is that the scoring range is highly compressed. The difference between scoring in the bottom quartile and the top quartile tells you a fair amount. The difference between the 90th percentile and the 99th percentile, where the vast majority of graduate students score, tells you very little. It is the difference of a couple of questions.