Upvote:4
Going by road from Gatwick Airport into central London is always a stressful and time consuming journey.
Depending on your actual destination (there is no single point that classes as "London downtown"), take the train
either
Southern (not the overpriced Gatwick Express) to Victoria, or change at Clapham Junction for Waterloo.
or
Thameslink to Blackfriars / Farringdon / St Pancras.