You've got three options:
-Apply now with deferred entry, I think all you have to do is put your start date not as next year but the year after. Make sure to double check with your university, and your department at that university that they do accept deferred entry. I was rejected by Edinburgh because of my deferred entry even though the uni as a whole seemed to be okay with it (The other unis I applied to were fine with it)
-You can apply now but defer your entry after you've gotten offers (wouldn't recommend this unless you've already started your application, there's no guarantee unis will accept you deferring)
-Don't do anything related to UCAS this year and apply next year once you know your grades
I've obviously only got experience of the first option but if you already know that you definitely want to take a gap year, and you don't think there's a chance of you changing your mind about the course or the uni I'd say that would be the best way to go, just gives you one less thing to stress about in your gap year when you've already got your place confirmed.