by train from gatwick to portsmouth

I thought train was out of Waterloo. Just look for the train schedule for London to Portsmouth. I think they are as frequent as every hour or more. Roughly an hour and half to two hours depending on the train. You will need to take a taxi from the train station to the ferry port and that will add another 30 mins to your schedule. I don't remember the company running that train. You might try trainline.com for information. This is an on-line ticket service that works pretty well.

You can look up all train times in Britain on the National Rail website: https://www.nationalrail.co.uk/

Gatwick to Portsmouth, two departures per hour:
xx:09, takes 1h36 with one change.
xx:39, direct takes 1h26.

@Frank, you don't need to go into London, there are trains from Gatwick. They start in London, Gatwick is an intermediate stop.
And the OP didn't say he was taking a ferry.

"You might try trainline.com for information. This is an on-line ticket service that works pretty well.!"

They charge a booking fee, the train operators don't. You can book through the National Rail website, it will direct you to the correct train operating company.

There is a railway station at Gatwick airport with direct links to Portsmouth, absolutely no reason to travel into London. I've used the service a number of times, it's a good, hassle free means of travelling to and from the airport particularly if you're unfamiliar with the area.

Book on the National Rail site.

See my reply to your other post about using aquacars.co.uk for door to door service to Portsmouth hotels upon your arrival at Gatwick.