I have the same dilemma. I need my fourth year to improve my GPA but then I would be having a year off, during which I am unsure of what to do.
I am unsure of whether I should stay back and do another year to redo and ace the courses I screwed up on or whether I should start my Masters (which would take up 2 years).
You apply one year in advance. If you take a gap year, you're actually technically taking 2 years off, because of the year that will pass while you are waiting to get accepted by schools. If you want to start dental school right after completing undergrad, you would apply in the summer between your junior and senior year. Hope that makes sense.