I don't know much about the Gessell test, but if you haven't done so already, do confirm with the tester or even your DS whether he completed the test and what he thought of it. If he wasn't impressed with the test he may not have put in his best effort.
One of my DS thought the test he was given for his kindergarten testing was boring, told the teacher who administered the test so and refused to do some of it. He still did well but obviously his test results had to be taken with a grain of salt.
