I'm an 4th year economics undergrad preparing start applying to PhD programs, and while I've never formally attempted to memorize GDPs, I've found that having a rough idea of where a county's per capita GDP is to be very useful in understanding world news and events (for example, I've noticed that around the $8,00-12,000 per year range seems to be the point where the median household gets an internet connection). If you do attempt to go the memorization route, be sure to use PPP-adjusted figures, as non-adjusted numbers will tend to systematically under estimate incomes in developing countries.