It is not a requisite of a gap year that they be filled with coursework.
Some people study for and (re)take the MCAT.
Some people do a post-bacc, masters, SMP, or even PhD program.
Some people work - as an EMT, a lab technician, or a waiter.
Some people voltuneer with AmeriCorps or Peace Corps.
Your gap year is entirely up to you what you do with it. I recommend taking a hard look at your application and ask yourself: what would give me the highest return for the time investment?
If you’re sitting on a 501, retaking the MCAT should be a priority unless you’re planning to apply exclusively to the lower- to lower-middle-tier DO schools. If your GPA is in the low 3s or 2s, an extended post-bacc or SMP probably fits you well. Low volunteer hours? Figure out how to get involved with Habitat for Humanity or Goodwill or some other community group in your area. All of the above? You may need more than one year, and that’s okay!
What you shouldn’t do is stack more classes into your application when you already have a 3.9 and have completed all prerequisite coursework, or be primarily involved in research when you already have a thousand hours (some exceptions apply).
Pick your battles well and have a productive gap year(s) and things will work out well for you!