Portsmouth, England ferry terminal to Gatwick Airport

One way is to take the train to Brighton, then train to Gatwick.

Or fast train from Portsmouth to London, changing at Clapham Junction for the Southern train to Gatwick. Dunno which is quicker.

There is a train once an hour from Portsmouth Harbour railway station to Gatwick, currently at 12 minutes pass the hour during the day Mon-Sat (different times but same frequency on Sun). There is another service at 29 minutes passed the hour which requires a change at Havant. Takes a bit under an hour and a half for the direct train and a bit more for the one with a change.