There is no "universities" when it comes to admissions. Different universities and different courses within them will do different things and use different criteria. There is no requirement for universities to follow a common procedure. There is also no requirement for universities or courses to make public what procedure or criteria they use to assess applicants.
Some will have an official scoring metric for GCSEs. This will be composed of some number of GCSEs, or all GCSEs, or GCSEs in some subjects. It might be used to rank applicants. It might produce a number that contributes to an overall score for each applicant. It might be used to exclude some applicants. Or it might not.
Some will use some GCSE grades, or all GCSE grades, to assess the likelihood of an applicant achieving their predicted A Level grades. Or achieving the standard offer for the course.
Some will look at GCSE subjects, or number of GCSEs, or spread of grades across subjects, to assess such things as well-roundedness, work ethic, ability to cope with high workloads or attitude to less favoured subjects to make an assessment of the attitude an applicant may bring to the course.
Some will have minimum requirements that mix university and course requirements in English, Maths and any other subject they think is relevant. Sometimes these minimum requirements will reflect the basic standard of literacy or numeracy the university or someone else feels are necessary for all students. Sometimes they will reflect the basic standard of literacy or numeracy required to succeed on a particular course. Sometimes they will just reflects a need to limit applications.
Some will consider GCSEs very important for some applicants, but not others.
Some will use GCSEs, in some kind of formation, to break a tie, at some point in the decision-making process.
Some will just feel, in some kind of abstract way, that GCSEs should matter as part of a holistic review. They'll think about them.
Some will not use them at all.
My course asks for 2 x A* and 1 x A and must find a way to reject two thirds of our applicants that doesn't rely solely on the uncertainty of a very narrow range of predicted A Level grades. For us, all things matter, in some way. For less oversubscribed courses, this would not be the case.