Gatwick Express Electrostar (UK)

These are relatively modern and smart trains, which are a especially adapted version of these commuter trains.

The three key on board adaptations are:

(1) power sockets at every seat, so there's no need to fret about charging up devices at the airport.
(2) additional luggage racks
(3) The seating is arranged 2 + 2 across the aisle throughout Standard/Second class, with space to store smaller bags beneath the seats

First Class seating is also arranged 2 + 2 so the key benefit of travelling First Class is that this area of the train will be less busy and you'll be more likely to store large items of luggage with easy access

Some of the seats in both First and Standard Class have tables.

Wi-Fi is available and wheelchair friendly spaces on the train and toilets have also been provided for.
If you require Assisted Travel the information is available here.

Seat reservations aren't available on these trains, so seats aren't absolutely guaranteed, though this shouldn't be an issue unless you will be boarding on Monday-Friday at Gatwick Airport between 07:00 and 08:30, or at London Victoria between 17:30 and 19:00.
That's because in addition to Gatwick Airport, these trains now also serve Brighton, which is a popular commuter destination.

Though these trains don't have dedicated bike spaces, non-folding bikes can still be taken on board as long as they are kept out of the way of other travellers.
However, you cannot take bikes on trains which are scheduled to arrive in London between 07:00 and 10:00 and that depart from London between 16:00 and 19:00 - more information is available here.