Can you retake maths GCSE any time?

No, you can't retake your Maths GCSE *any* time. There are specific exam series throughout the year run by the exam boards (e.g., AQA, Edexcel, OCR). You need to find a centre (usually a school or college) that's offering the GCSE Maths exam in one of those series and register with them in advance of the exam date. There are deadlines for registration, so it's not a spontaneous thing.