Budgeting time to get from Eurostar / St. Pancras to Gatwick

There is a Erostar train that leaves Paris at 11:13 and arrives St. Pancras at 12:39. You will go through security in Paris. Once you arrive in London you exit the train and are on your way to the tube station.

According to TFL website, it takes 8 minutes to get from King's Cross St. Pancras Underground Station to Victoria station.

The Gatwick Express takes 30 minutes to get to Gatwick airport.

For International flights I like to arrive 3 hours prior to departure.

So if it were me, I'd try to catch the Gatwick Express that departed no later than 13:30.

The times I mentioned are best case scenarios. There could be a delay with the Eurostar, or the Tube or the Gatwick Express

You can get a train direct from St Pancras to Gatwick. Trains for Gatwick (final destination Brighton) leave from the "Thameslink" platforms which are under the main station. This will be a lot easier than taking the tube (not the 'subway') to Victoria. Times here: http://www.nationalrail.co.uk/
Both French exit and UK Passport entry controls for Eurostar passengers are done at Paris Gare du Nord, before you get on the train, at St Pancras you just walk off the train like any other train.

Check how long you need to check-in at Gatwick with your airline, it varies depending on airline and destination.

Thank you for the replies, I booked so that I arrive in St Pancras almost 5.5 hours before my flight leaves, hopefully that will leave plenty of room for things to go wrong and still make the flight.

And of course you gain an hour. Yay !!

Thameslink takes about an hour from St Pancras with 4 departures an hour (weekdays) so all going well you should be at Gatwick about 4 hours before your flight time.