Agreed - train very definitely the best option, with good connection, it's generally around 40/50 minutes.

I drove down to Gatwick on Friday morning to pick up my sister but only because she was arriving in at 8am and was travelling with 2 small children and we didn't fancy travelling on the train at rush hour with toddlers.... I did the journey in 50 minutes but that was a good run and it was early morning. It took us about an hour on the way back, but again, there were no traffic issues, which you can never count on.

My other tip on taxi/driving to Gatwick - go through Bromley, so A21, M25, M23 - longer by distance but very definitely quicker (at least, it is if you live on the Perry Vale side of Forest Hill).