The figure represented an additional 274,000 passengers, and the gateway's busiest ever September.
Long-haul saw the highest growth in September, and North Atlantic traffic, which has recently been bolstered with new routes to New York and Los Angeles, increased by 12.6% while travel to other long-haul destinations was up 12.5%.
This included a 5.6% increase in passengers flying to Vietnam and a 12.4% increase on traffic to Dubai, while Gatwick’s links to the east saw an additional boost in September with the launch of Garuda Indonesia’s new Jakarta services.
A key segment of passenger growth at Gatwick continues to be business travel, and this was particularly notable in European scheduled traffic, which it says was 11.6% up - or 234,400 passengers.
Growth came from business routes including Paris and Amsterdam, as well as recently introduced routes to Berlin and Warsaw, and Gatwick saw an increase of over 250,000 business passengers on the prior year in September, with one in five passengers now travelling for business.
Nick Dunn, chief financial officer at London Gatwick, says: "Gatwick continues to prove that competition in the London market is working for passengers and airlines. More people than ever are choosing Gatwick over other airports and new short and long-haul routes continue to open and prosper.
"In September, we have seen significant growth on the UK’s only direct route to Vietnam and the start of a new service to Jakarta from Gatwick.
“Building a second runway at Gatwick would enable the benefits of competition to continue to flourish. We must look to the future when it comes to making a decision on where new airport capacity goes and Gatwick is the only airport that can provide the balance between low-cost short-haul and long-haul growth which can deliver economic benefits to UK PLC from 2025."