Transport Minister Baroness Kramer, was on site to open the platform, that is aimed at delivering greater reliability for airport passengers and commuters.
The project was made possible after a joint campaign between Gatwick Airport, local business groups and airlines, with Gatwick contributing €8.5m to the project to ensure the upgrade could take place.
The new platform and section of rail line will allow fast trains from both London Bridge and London Victoria to travel through the station more quickly.
Signalling upgrades will also allow for more reliable travelling times in preparation for service improvements associated with the Thameslink Programme upgrade, which will increase the number of trains and range of destinations from Gatwick from 2018.
Stewart Wingate, Gatwick's chief executive, says: “London Gatwick is already the best connected UK airport by rail and this upgrade will benefit the broad range of people that the airport station attracts, from tourists to the business traveller. It will allow us to compete even more for passengers and airlines wanting to come to London and the south east region.
“This is the first step in our vision for a further major redevelopment of the airport station – the Gatwick Gateway. We recognised with over 14 million people arriving, or departing by rail, that an investment had to be made to improve the passenger experience in and around the airport.”
The opening is the beginning of a rail improvement programme that will see ongoing upgrades over the next four to five years at Gatwick Airport station.
The Thameslink programme will provide an additional 50% capacity by 2018, following the introduction of new longer trains and infrastructure improvement.
Of the 35 million passengers using Gatwick each year, more than 13 million arrive or depart by rail.