This is something I've looked into a lot recently, since I decided to take a gap year a couple of months ago. The consensus from everyone I've asked is that it is highly recommended that you keep some connection to medicine, but you don't have to spend the entirety of your gap year doing so. If your goal is to make money for med school, that's a perfectly valid justification and shows you are being practical about the costs. Take whatever job pays you the best while leaving you enough flexibility for the application process. You can volunteer a few hours on the weekend to stay in healthcare. If you look through older topics on this board, you'll see stories of people doing everything from bartending to research to volunteering.
If you do want something healthcare related, look into Americorps' Health Corps program. It doesn't pay the best, but you stay in the health field and get something you can talk a lot about at interviews. And if you've built up a good med school resume, you shouldn't have too much trouble getting a spot. At least for me, that's the current plan (still have a year of undergrad left).