The security lockdown comes less than 24 hours after over 120 people were killed in coordinated terror attacks across seven different locations in Paris.
Hundreds of passengers were escorted outside the airport at around 10:30am.
A spokesperson for Sussex Police said: "The terminal is being evacuated at the moment as a precaution. It is for the protection of the public.”
In a statement Gatwick Airport said the evacuation was a "precautionary measure" and it had provided "extra staff on-site looking after passenger welfare".
Huge crowds of affected travellers were photographed outside the terminal on Twitter.
British police are believed to have tightened security measures at major British air and sea ports following the atrocities in Paris - the deadliest terrorist attacks since the 2004 Madrid bombings.
Metropolitan Police Commissioner Mark Rowley has said the current threat level will be reviewed at a Cobra emergency meeting with David Cameron this morning.
It is currently at "severe" - the second highest level meaning an attack is "highly likely".