I fall into the category of writers who have an "ear" for grammar without having successfully memorized all the rules. I blame my upbringing--at the time most of my peers were learning English grammar, I was learning French (it was assumed that I already spoke English, and while we had one class session a day for English language and American history).
Of course, that was over by fifth grade, at which time I was stateside again. But it's my story and I'm sticking to it.
Anyway, a writer who does understand grammar is Mssr Crankypants, who explains gerund usage here. I'll be visiting the site often, and recommend it to anyone else who writes, or wants to. Especially if you don't have that "ear." Of every aspect of writing, grammar (and okay, spelling) is one of the few that can be learned, which means there's no excuse for getting it wrong unless you're after a specific effect (and to achieve that effect you still have to know what you're doing).