4 A*s is excellent, but they do turn down many US applicants with close to perfect scores and grades. They will look at the GSCE. However, I would be more concerned about getting US test scores. Harvard/Yale's average SATs are about 2250 and that includes students with "hooks". It would be best to get 2300+ or 34+ on the ACT. The SAT Is are weird from your perspective, but you need to prepare for them. Also, best to have 3 SAT IIs at 800 or close. For US applicants it helps to be a talented athlete, musician, actor or something, or have done something noteworthy nonacademic. They will know you won't have the opportunity for US-style ECs or be familiar with US tests, but it is better if you can present US-style credentials. Also, it is important how you sell yourself in your essays and how your faculty references present you.