Eight hours is a long time to spend in the airport, while also not being quite long enough to see much of London. The Gatwick express does run regularly, and takes between 30 and 45 minutes to get to London Victoria. However, there is a significant amount of construction going on right now that will alter the timetables. So you would want to check those carefully are the dates that you're traveling. Upon arriving at the airport, you'll need to clear customs and immigration, and make your way to the South terminal. This is where the left luggage hall is and also where the Gatwick express is located. The area around Victoria station is walkable, you can easily go to Buckingham palace and Green Park, or wander around Belgravia. Also, close to Victoria station is Westminster Cathedral, not the Abbey which is further, and the Cathdral is an interesting example of Byzantine architecture. You'll want to keep an eye your time though, as it's recommended that you are back at the airport three hours before your flight. If you wanted to stay close to Gatwick, the town of Horley is kind of interesting. They have a lot of traditional architecture there.