Here are some interesting articles (1, 2) on how students who’ve studied Latin tend to score higher on the SAT verbal sections. Although not applied to the GRE, I would think similar results could be expected.
Unfortunately, the studies don’t prove that studying Latin is causal–that is, that learning Latin will increase your score. They only show that those who’ve studied Latin score higher. Perhaps students who take Latin tend to be students with stronger verbal skills initially?