I think you're confusing two different things:
1) There are one year programs, called special masters programs, where you take medical school classes with medical school students. Since these programs show your ability to succeed in medical school in the most direct way possible applicants who do well in well known SMPs almost always get into medical school on their next application cycle, which makes them amazing options for people that graduate with lots of credits and low GPAs. However, unless you were already a marginal candidate, you do basically need to wait until the end of the program to apply, so you get a new gap year after the program. A few programs save you the gap year by taking almost all of the successful applicants from their program into the next years medical school class, these are called high linkage programs and there aren't many of them. That might be what you're thinking of.
2) If all you want to do is fill time in a gap year, a lot of people will recommend a 1 year MPH. This is NOT to improve your GPA. The goal is instead much more farsighted: to improve your residency applications and your chances in academic/administrative medicine. It's not the only option for your year by any means, but it's not a bad option either. However, again, the focus here is NOT on your medical school application.