The easiest way is probably by coach with the journey typically taking 50 mins to an hour. With road works or heavy traffic on the M25 motorway it can take longer. National Express is one option and The Oxford Bus Company is another.
Travelling by train, catch the Elizabeth line train (see TFL map) to Farringdon, change to a Thameslink train which stops at Gatwick airport.
Travelling by train, catch the Elizabeth line train (see TFL map) to Farringdon, change to a Thameslink train which stops at Gatwick airport.