Don't worry OP, many people take gap years either because they didn't get the required AS grades (like me) or because they were rejected by all 4 choices. It doesn't disadvantage you at all providing that you make good use of that year to carry out voluntary work, work experience, etc.
A well spent gap year would actually put you at an advantage as you'd already have your grades, would be able to get tons of experience and reapplying shows immense commitment and dedication on your part.