im not 100% sure how high a weighting gcses have on the Cambridge application, but i am very certain that Cambridge weighs the entrance exam (and even more the interview) over GCSE grades (they understand that people could be very good at one area i.e., maths and weak at another subject i.e., languages) and a level predicted grades (cos alevel predicted grades can be very subjective).
regarding gcses and ur overall application, they heavily consider your contextual status (i.e., what school you go to, any extenuating circumstances etc..) when weighing your application. no one rlly knows how the Cambridge admissions process works (i am a current offer holder for physical natural sciences from a fairly good grammar school and am noncontextual btw), but people who get in tend to get gcse grades which are straight 9s or close to straight 9s. this could be a correlation, not causation thing (people who get those grades grind a lot and thus a good fit for Cambridge).
however, since you are applying for economics, which is a very heavily maths-based course, not getting a 9 in gcse maths could heavily hinder ur application (unless u have contextual circumstances explaining why u didn't get a 9 in gcse maths)
best of luck with ur application!