The raider struck at about 7.30am as a blue Brink's Limited security van was making routine collections and deliveries to airport shops and was parked in the south terminal service tunnel.
One of the van's two-man crew was on his own in the vehicle when a lone perpetrator attacked the van, Sussex police detective chief inspector Mike Alderson said.
He said: "Early indications are there was one perpetrator. One person was away from the van and whilst he was away the van was attacked and a significant amount of money was taken."
He said no weapon was believed to have been involved. The victim, a man aged in his late 30s or early 40s, was punched in the face. He was taken to the East Surrey Hospital and treated for a suspected broken nose.
Det Ch Insp Alderson stressed that the incident did not happen in a security controlled zone within the airport.
He said police were examining CCTV footage and speaking to people who witnessed the incident, which lasted a matter of minutes.
The suspect was described as white, in his 30s with a goatie beard. He was wearing a light blue shirt, dark blue trousers and had a helmet over his head.
An airport spokesman rejected the idea that the robbery showed security at Gatwick was not tight enough.
The road in question has been closed, he said, adding: "At the moment, it is having a minimal impact on operations here in terms of passengers arriving at the airport, and flights are getting away on time."
In 1983, a gang of criminals stole three tonnes of gold bullion worth around £25m from a Brink's-Mat security warehouse at Heathrow airport.
