Surprised Gatwick is very low on the list, well served by train I think.
Only when they actually run! Gatwick deserves its low place. The line from London to Brighton is a disaster area. I pass through Victoria station weekly and almost every time I hear announcements "Gatwick Express are sorry that the xx.xx has been cancelled". Likewise, the normal trains are delayed or cancelled. If it's not a train breakdown, it's the weather, or signalling problems and, last week, a new low, several trains were cancelled due to "crew shortages".
On a few occasions the entire line was not operating for one reason or another. Twice, in getting to LGW I had to go out a share a taxi with others to ensure catching a flight. The lack of information is deplorable and you can't take a chance that the service "might resume in an hour or so".
A train service connecting to an airport needs to be reliable. From my experience the Gatwick Express is not so I rarely fly from there anymore.