A different direction to Heathrow
Gatwick takes a similar time to Heathrow but heads the opposite way. From Brockenhurst we join the M27 eastbound, continue on the A27 past Chichester and Worthing, then turn north on the M23. It is around 93 miles and rather less of it is motorway than the Heathrow run.
The A27 through the Worthing and Lancing stretch is where this route loses time in the morning peak. That single section is the main reason a Gatwick departure sometimes needs an earlier start than the mileage suggests.