As most flights coming from west, always arrive ahead of schedule. most likely you will be out of the terminal at Gatwick before 10am, which is the peak times on the M25, one of UK busiest motorway. few here on flyertalk reported that it took them just over 2hours and half to get from Gatwick to Heathrow or vice versa.
If you travelling during the weekdays, consider taking the Gatwick express into London, the train takes 15min to arrive into Victoria Station. then a taxi or the tube (6stops) to Paddington Station to catch the Heathrow express that takes 22min to Heathrow. And if you arrive early to Paddington Station there's great coffee shops around to relax and catch some fresh air. trains are much more frequent then national express.