Awaiting Approval by GAO is the final stage of the Cambridge admissions cycle. It means that the department has decided to give you an offer. BUT at Cambridge the Graduate Admissions Office (GAO) has to verify certain things before you can officially receive an acceptance, mostly to do with making sure you didn't commit fraud
So congratulations! Barring something truly catastrophic (or you actually did commit fraud lol) you should be receiving an offer within 10 business days (might be longer so don't panic until about two weeks have passed).