I took a gap year between high school and college to volunteer in central america for about a year. I've always wondered if I would be able to include that on my application since I wasn't in college yet, so, will I?
You can. You include everything that you did starting from the time you graduated from high school, and everything after that. Plus, medical schools are going to want to know what you did for that year anyway, so it serves in your favor to include your volunteer work in your application.