I would suggest you go down to Gatwick the night before, using the coach as suggested already.
The route you take can largely depend on how many travelling. If you were to get a taxi, with 5 Star Taxis, or similar, the cost is £55.00.
The full public transport option would be by train, from Windsor & Eton Riverside train station to Clapham Junction, and then take the Gatwick Express. You would need to take the 05:53 train from Windsor, with a quick connection at Clapham Junction that would have you arrive at Gatwick at 07:07
For unfamiliar journeys, I would always look at www.rome2rio.com for options