Hi LanadelSlay. Is it a social science or humanities subject your DD is applying for?
If so, a gap year can be a brilliant idea.
They need to go to uni feeling positive, I think, not that they are unsure or compromising. DD was in a similar position to your DD. What you find is, that once they are on the gap year and doing something purposeful (particularly if it's overseas), they gain a totally different perspective anyway. They realise that their other options (even if Oxbridge doesn't work out) are great and they own them positively. It's a different mindset once the pressure is off and they're out of the school bubble. They don't even need to tell anyone they are reapplying.
Have a look at organisations such as Op Raleigh or GVI to get an idea of things she could do in the gap year. When they start at uni after doing that kind of thing, they are significantly more mature and take it all in their stride. This is a massive benefit wherever they end up and, not only that, they have memories and experiences that last a lifetime.
And yes, I do think there is an advantage to reapplying with grades in hand (as long as these are very top grades). It's a steep learning curve first time round, but much easier second time. She will need to do something extra though - like essay competitions - to boost her application. In one of DD's interviews, they asked a lot about her work abroad and essay competitions she had entered / won.
Bear in mind, that if she gets an offer next Jan it will be unconditional, so then she can relax for the rest of the year, knowing what her plans are. By the time August and A-level results roll around, Jan does not seem far away.
A gap year can be amazing, but the key is to have a clear plan.