GCSE results determine whether your daughter will get an offer from a university. If she has good GCSEs she is likely to get offers from better Universities. This offer may be AAA or ABB for example. This means your daughter will have to make these grades at A level or her offer is withdrawn. So they're both very important.
But when your daughter does a level she will be choosing 3/4 subjects in which she is good in, so if the teaching falls short she can pick up the extra work herself. GCSEs will cover a wide range of topics and many Universities require As and Bs in foreign languages, English, maths and science at GCSE and if your daughter does not score as highly on these subjects that is problematic.
I would argue its far more important to have good GCSE teachers than A levels and the better the GCSE scores are the lower the offers she will receive taking off pressure at A level.
mkmo kindly, this is rubbish. Offers for university are based on predicted A level grades. Only a handful of universities look at GCSE grades (Oxbridge mostly). Therefore it is dangerous to say that applicants have less pressure for A level grades if the GCSE results are good. A level syllabuses are hard, and do require good teaching with very little room for students to “pick up” the learning themselves.
Offers for any particular university course will be a standard for all applicants regardless of their respective GCSE grades - you imply that offers will be lower than the norm if GCSE grades are better. Some universities will make lower offers to students at particular schools (contextual) or unconditional offers but this has nothing to do with GCSE grades.
The main link between GCSE and A level is that you need a minimum grade in the GCSE to study the corresponding A level (or a similar subject if the A level is a new subject). A high grade at GCSE might be an indicator of likely A level grade but often is not, as the jump in level can be quite marked.
So, in short, both GCSE and A level performance is important for different reasons. But not those voted by Mkmo