You can contact the university you have firmed and talk to them about your decision to take a gap year. However, I wouldn't really recommend a last minute gap year (not to say that I wouldn't recommend a gap year at all, just make sure you're making the right decision). Definitely research what you're wanting to do -> How you're going to fund it, job options etc.
Research some gap year projects, as well as think about what you'd want to do for the gap year. Is it a time of travelling, festivals and such? Or do you plan on building up more work experience and potentially getting in to a higher university (or making sure the course would be right for you)?
Perhaps start looking and applying for jobs in the few weeks before results day so you definitely have a job and some money coming in. Even if it's not a job you particularly want, even just stick with it for a while to get money in to ensure that you can afford all that you want to do in your gap year!
I'm currently on a gap year of sorts and working before I go to university this September. While I am still doing work, it's allowed me to get money to go to festivals (abroad and in the UK) and on holidays. Whereas when I didn't have a job for the first two months, I was stressing about applying for things and getting money coming in because I left applying for jobs too late, and realised that applying for jobs isn't always the easiest!