Dev Studies is probably a good one though, as you should have less difficulty making your course relevant to "the development of your home country" (showing your "commitment to benefit others") - that's what the Gates application form asks you to do - than someone studying Mediaeval Ebglish Literature or something like that. You should definitely apply - most people do (hence the competition is quite tough) - and you can probably also apply for an ORS grant through your Faculty. Depending on which college you'll be at they might be able to help you out as well, so bear that in mind when making a decision (I don't know whether you've given any thought to college choice yet?)
Remember, however, that the deadlines are early: Gates is February 28th (or November 1st, I think, if you're American), ORS is February 1st. If you want to apply for Gates that also means that your application for your course has to be in by October 15th, I believe. Well, that means you've still got half a year, which is plenty of time to be fair.