To gratify is to give satisfaction. So if you've managed to get straight A's in school, what you did was gratify your parents — and it probably felt pretty gratifying to you too.
There's a strange taint to the word gratify. Sure, it means to do one's best to please, like the little straight-A student that you are, but there's also a hint of indulgence about the word. How often have you heard the phrase "gratify one's desires" used disapprovingly? Slightly hoggish, a bit self-indulgent, that's gratify for you.