Weekday rush hours are the main thing to plan around for Gatwick to Portsmouth, since the M23 and M25 get busy with commuters as well as airport traffic. An early-morning or early-evening weekday trip is worth a little extra buffer; things generally ease off in the evenings, at weekends and during school holidays. The fare stays fixed regardless of which of these you land in.
Your pickup point is confirmed ahead of time, removing any guesswork on the day, and flight tracking on airport journeys means the car adjusts to when you actually land rather than when the schedule says you should. The usual airport steps still apply either side of your drive — check-in and security, or passport control and baggage reclaim — and the included waiting time is there to cover them.
Groups and luggage are worth mentioning at booking rather than working out on the day: a saloon suits up to four people and two large cases comfortably, while an estate, MPV or 8-seater gives more room for extra bags, buggies or sports kit. School holidays and the summer season tend to bring more group and family travel on this kind of journey, so booking a little further ahead helps guarantee the right-sized vehicle at a time that suits you.