Actually, the GRE is partly an achievement test. But, I chose the above title because I believe that thinking of the GRE as an achievement test doesn’t help much, and can even hinder your preparation efforts.
An achievement test is about what you know, as opposed to how you problem-solve using what you know. This is because the majority of the questions have twists and turns, like spaghetti, and it is our job to unravel it. So, as we study and practice, we learn how to unravel it, learn some strategies, and create a game plan.
It can help tremendously to adjust how we think about the test. (It’s not just a perceived barrier to acceptance into the graduate program of your choice.) What is the test trying to do? What does it (the authors, actually) want to know about me and my thinking skills?