I don’t do medicine, but I took a sort of gap year (I resat two a levels, and worked full time) and I can confidently say it was a great decision. I had the opportunity to get my grades up, and save money for university. It also gives you time to think about any future options, travel etc.
ultimately if you feel like there’s going to b a disparity between your predicted grades, and what you actually achieve 100% take a gap year and apply with your achieved grades. I was rejected by a uni I wanted to go to because of my predicted, but I ended up doing better than what they asked for lol.