There were so many caveats in my answer supra that it should be clear that I am NOT saying that IQ scores and GMAT scores are causally related in a meaningful way (after all, I called my own answer facile). However, it is likely that the scores are correlated.
I am strongly opposed to the idea that one's score on the GMAT is fixed. I have seen students experience very large increases in their scores on the GMAT and many similar tests.
People often forget that standardized tests can be of any format as long as that format is administered consistently across all subjects. A standardized test can consist of essays, multiple choice, or short answer formats among others. The fact that the time and format are enforced across students is hardly unique to standardized tests.
Regarding whether the GMAT is beatable- the GMAT, like virtually every classroom exam, is "beatable" in the sense that students can study hard and answer more of the questions correctly. However, as we observe with classroom exams, many students fail to put in the effort to receive a higher score and others simply do not understand the material.