Hiii, it's definitely possible to be self teaching a GCSE because I'm doing just that, and for 2 of them. If you are determined enough it is definitely possible, but it is quite difficult. I am only able to do it because there are teachers in my school who are helping me outside of lessons, for around half an hour each week. It might be difficult if you don't have a teacher as such because otherwise you might not understand the exam technique or have anyone mark essay-style questions in the economics papers. One of my friends take Edexcel and they seem to quite enjoy it, but my advice would be to look at some exam boards and the specification, find one that you enjoy the most and then get hold of a textbook.