Well, it depends what the GCSE is in. If it's maths or english you may be able to take an equivalent course - Learn Direct run some that you can do in a few weeks. But you need to meet the conditions of your offer by Aug 31st, so you'd need to start looking into that now rather than waiting til you know you've missed your offer. You'd also have to contact the uni, tell them you're doing the new qualification, and ask them to amend your offer.