If you get the required grades (AAA), and your UKCAT/BMAT etc is good enough to get interviews, then I don't see why you would be rejected just because you took a gap year.
But yes some medical schools would like to see that you built on your work experience during the gap year.
If you mean that you want to resit your A levels, then off the top off my head I am sure the majority of medical schools won't accept any resits. There might be a few, it's just a matter of doing your research.
As long as you get the required grades in your first attempt, you won't be disadvantaged if you took a gap year.