Many years ago--business grad school--one of the professors (forecasting?) had done research into GMATs and various post-school success metrics. Like it or not, the standardized test scores were a better predictor (by far) than anything else.

In terms of gaming scores on standardized tests generally, yes people with more money can take prep classes and the like, but there are also test prep books available--presumably even from the library--that probably get you a lot of the way there.