I had a lunchtime transatlantic flight from Gatwick north 2 weeks ago, and the queues were very long at 9am, but moved reasonably quickly (about an hour). However, I stayed in the Hilton Gatwick the night before, and had a wander round the terminal between about 8 and 9pm, and it was empty, with no queues at all for security. I would still turn up early, but be prepared for a pleasant surprise when you turn up (i.e. bring a book to read in departures!).