Sorry this deal won't be for the majority but hopefully it will help someone flying out of Gatwick. It has been around a while but couldn't find it on the forums.
First Great Western have a special deal, called the Gatwick Flyaway, on their trains for passengers who can provide evidence of a flight booking from Gatwick. The return fare depends on where you travel from but the good news is the ticket allows up FOUR adults to travel for the same fixed price starting at £40.
Plus up to 4 children (ages 5 - 15) can also travel accompanied for an extra £1 each.
The ticket is an open return and you can even travel peak period. No travel via London is permitted.
For example, four adults and four children can travel from Swansea for a grand total of £94 return. See the link for other departures and details.