There's probably not any one answer for this, but based on your individual experiences, how long has the queue been for you at immigration at Gatwick for the morning US arrivals? I'm on a flight that arrives around 8:20 AM the Saturday before Christmas and I'm wondering how much time to allow between arriving and getting the Gatwick express into central London. >2 hours from arrival I'm sure would be smart. (I'll be booking a ticket for that asap.)
Last time when I flew into Heathrow we were the only arriving flight so everyone was through passport control in less than half an hour, maybe less. Fantastic that was!