There's no single set of GCSEs required to become a teacher. The specific requirements vary depending on the country, the subject you want to teach, and the university or training route you choose. However, generally, you'll need a good range of GCSEs, including strong grades in English and Maths (usually at least a grade C/4 or equivalent). Many universities will also expect GCSE science subjects if you wish to teach science, for example.
In short, you need to check the entry requirements of specific teacher training courses (whether that's a PGCE, SCITT, or other route) at the universities or institutions you're interested in. Don't assume anything - always check directly with the provider.