Armed police already on patrol at the terminal were alerted to the Frenchman's 'suspicious' behaviour after he was challenged at the check-in desk around 10.30pm (NZT).
It was unclear whether the man had intended to use the guns at the airport or to try to board a plane with them, a source said.
An anonymous source who works at Gatwick added at the time: 'Our officers are treating this as an act of aggression.'
Senior officers from Sussex Police were called to the airport's police station and were understood to have convened a 'Silver' control meeting to discuss the security breach.
A 100-metre cordon was then erected and bomb squad officers were called to the scene to examine the weapon that had been discarded inside it.
As a precaution they carried out a small controlled explosion, it was said.
Detective Superintendent Nick May said: 'The man is being interviewed as we try to determine the circumstances of the incident, but at this time it is too early to say what his intentions, if any, were.
'However, given the events in Paris on Friday evening, there is heightened awareness around any such incident and it is best that we treat the matter in all seriousness.
'We are aware that there is concern about what has happened in France, but the general threat level remains the same and people should be aware as usual of anyone acting suspiciously and report any concerns immediately.'
Sussex police added in a statement: 'Personal items and what appears to be a firearm were recovered and have been removed for forensic examination. However, the viability of the weapon has yet to be established.'
They also stressed that the Frenchman was 'landside' at the airport and had not checked in or passed through any passport or security checks.
Eyewitness Tim Unwin, a geography lecturer at the University of London, tweeted earlier that the terminal was in a 'shutdown situation'.
It is understood that passengers were moved to Jubilee House near the North Terminal before being transferred to a Sofitel hotel.
However, there were complaints about conditions.
Mr Unwin added: 'Very cramped in Sofitel - passengers are now beginning to get frustrated - would like to be given some information.'
Passengers due to land at the North Terminal arrived at the South Terminal during the evacuation. The North Terminal was re-opened at around 5am (NZT), around six hours after the lockdown began.
A Gatwick Airport spokesman said: 'Following a full and thorough search by the police, we are now able to re-open our North Terminal to passengers.
Passengers should check with their airlines for information about their journey.
'Our priority is always the safety and welfare of our passengers and extra Gatwick staff are on hand to assist those who have experienced significant disruption to their journey today.
'We are working with our airlines to return North Terminal to usual operations. South Terminal is operating as normal.'
The drama at Gatwick followed terrorist attacks in Paris that have left 127 dead.
- Daily Mail
