At least 19 incoming flights were diverted and dozens more delayed at the airport in the south of England.
"Arriving and departing flights recommenced from Gatwick at 11.38GMT. The runway closure has caused some flights to be diverted to other airports in the area and there will be delays to some departing flights," said a Gatwick Airport spokesperson.
"Friends and family waiting for passengers should contact the relevant airline for the latest information."
Details of the spillage, and its cause, have not been disclosed, but reports suggested oil had leaked from a Boeing 747-400 plane jet on landing.
It is London's second-largest international airport and the second-busiest in the United Kingdom after Heathrow.