Surrey Hills are alright but busy and Box Hill is over-hyped unless you like that sort of thing.
Those that like a hilly challenge could find better climbs in the North Downs and Greensand Ridge, or there's always Ditchling Beacon on the South Downs.
Or for a gentle ride through lots of farmland, woodland, a reservoir and viaduct, consider this circular route:
https://www.plotaroute.com/route/1599160
I've cycled much of that over the years, but never as a circuit, so I've only just thrown that route together.
You could start from Gatwick, but the roads near the airport are busy so it would be more enjoyable to drive past Crawley and start the ride from further south. It's about 30 miles but could easily be trimmed.
The forecast is for light rain on Sunday, if that matters.