Personally, I would retake the GCSE alongside the access course. It might be a good idea to speak to uni admissions about your current gcse grades as some universities won't accept people with lower GCSE grades.
My situation is a bit different as I retook my gcse during COVID and then completed an access to he after
It depends on which universities you're looking at applying for if you can get into a couple of universities that you're happy with then I wouldn't bother retaking any gcse