Be there no less than 3 hours before. 7:30, or a little earlier.
Depending on how organised you are, and if you have some credit left on your Oyster Cards, either walk to Maida Vale or Warwick Avenue tube stations and take the Bakerloo Line to Oxford Circus - easy cross platform connection to the Victoria Line to Victoria (I'd pessimistically allow half an hour for the tube) and then upstairs into the station and take the Gatwick Express (or if you have a few minutes longer and a few fewer pounds to spend, take the Southern service) straight to Gatwick. It goes frequently, and is about half an hour. So I'd want to be walking into the Maida Vale tube station - or Warwick Road - about 6 am. At that time you are ahead of the rush hour, and the rush hour is lighter and later on Friday mornings.
OR
Take a cab from the flat to Victoria and then as above. You might save half an hour but spend more money, although with 4 of you and luggage you may come out ahead.
DON'T use the cab all the way to Gatwick. There's no way it can go on the A23 and other rat runs with 30 mph speed limits and loads of traffic anywhere near the 125 mph that the train reaches, and the fare would fry your whiskers.(or hubbie's).