From the DfT :

The Department for Transport today announced a package of improvements that includes expanding the Gatwick Express service, building new trains and leasing more carriages to provide more than 12,000 extra seats. Service changes and capacity improvements will be in place by December 2008.  They include:

The Gatwick Express will be retained as a non-stop service between Gatwick Airport and London every 15 minutes. Premium, fast services for air passengers will be retained, as will the current timetable. At the busiest times of day the service will also run beyond Gatwick to Brighton, doubling the number of express trains between Brighton and London Victoria in the high peak.
 
An extra 48 carriages will be made available on the Thameslink route operated by First Capital Connect. The first 16 will begin operating on the line, which runs from Bedford to Brighton through Central London, by the end of 2007 with all 48 in place by December 2008. The extra coaches will be transferred from Southern.

Southern will replace these carriages with 48 new Electrostar coaches (the same model as the rest of its fleet).

The current Gatwick Express franchise will end in May 2008.  All services will be incorporated into the Southern Franchise to integrate the service changes, which will be implemented in December 2008. 

More details can be found in the link below.
http://www.gnn.gov.uk/environment/fullDetail.asp?ReleaseID=276432&NewsAreaID=2&NavigatedFromDepartment=False