Some universities accept a GCSE maths equivalency test for a nursing place, some run their own, and at least one says no. What the test is, who accepts it and what to ask before you book.
Functional Skills L2, Access to HE, NHS Numeracy, Adult Numeracy: what’s accepted where? A clear comparison for nursing, PGCE, police, armed forces and university entry.
The honest answer to what GCSE Maths grade you need for nursing: Grade 4 in almost every case, Foundation tier is enough, and how long it takes from where you are.