Hey, so as you probably know already there's some other exam you need to take (I think they're called UCATs) for medical school applications. A friend of mine didn't do so well in their UCAT exams so decided instead of applying and putting pressure on themselves to go through the application process this year (with grades they didn't think were good enough to get in), they would take a gap year and take the UCATs again. Medical school is a HUGE commitment so it may be good for you to take a break before going back into education but it's different for everyone - don't feel pressured to take one or not to take one just because someone told you to/not to!
Gap years are not a waste of time as long as you use your time and don't sit around doing nothing. You don't necessarily have to do anything related to your chosen career; you could travel, work a part time/full time job, take some time for hobbies, enter essay competitions etc. If anything, it can often "show initiative" to take a gap year and start gaining work experience, as I've been told this is what has been said to people I know by employers in job interviews.
You could apply and if you don't get in, take a gap year and try again the next year, or differ your place if you do get in and decide to take a gap year anyway.