Retaking GRE during gap year from PhD to improve pre-doctoral fellowship award chances
The return of investment on a GRE score is much less than other activities. If your score is horrid, then go for it. I wouldn't spend more than 2 weeks prepping, though. Squeezing out every point from that GRE is not worh the effort. I would wager that the score is only to determine the seriousness of your application, and that it would not be a major deciding factor otherwise.