I taught languages for years in secondary schools and worked as a private tutor before finally retiring.
A number of my private tutees took GCSE as private candidates. The exam boards (AQA and Pearson Edexcel) can give you advice about local examination centres and whether you need to find your own oral examiner - sometimes they provide them, but you'd need to discuss that with the centre well in advance.
As others have written, there are loads of general resources out there and it depends how much experience you have of learning a foreign language and, of course, your motivation whether you could do it on your own. Nearer the exam, I would advise finding a good tutor who could prepare you for the requirements of the exam. GCSE languages are changing this year, so nobody has any experience of the new requirements and there aren't any textbooks published yet (I believe they're due out in June), but a decent tutor would have made the effort to find out what's needed.