To add to what you have already been told...
Bookmark this Link It's a link to the National Rail timetables and ticket booking site.
You will see that you can get from Gatwick Airport to Dover with one change at Tonbridge Wells in either 2 hours 17 minutes or 2 hours 36 minute depending on what time you depart Gatwick. This would be faster than going into London to change to the Dover train.
There are also fast express rail services Heathrow airport (LHR) to London and Gatwick airport (LGW) to London, if you want to spend a few days in central London before or after your cruise.
By road Gatwick to Dover is about 80 miles by motorway (freeway) - 1hour 15 minutes drive. A one way car rental, a limo service, or even a taxi might work for you. If you take local roads the journey would be shorter, much more scenic, but would take longer. (there are taxi/limo companies based at Gatwick, just as there are at MCO, that will meet you off your flight and take you to Dover or to London for an agreed fixed price)
If you are using a Disney provided transfer, check with them first as they might be picking up guests from Heathrow as well as Gatwick.
According to the itinerary for my next trip to WDW, I leave MCO at 18:05 on VS08, arrive LGW 06:55 I would allow a maximum of 2 hours for baggage reclaim, immigration & customs so you should be on the road by 9am and in Dover before lunch.
On your return to Orlando, the flights leave earlier in the day, 11am and 1pm, with at least two hours before that for check-in, security & passport control. I always arrive Gatwick the night before and stay at one of the two airport hotewls (Hilton or Sofitil) Hilton is most convenient for Virgin because it is at the right terminal.
Hint ... they have beds in the 'pointy end' of the aircraft so you arrive with the minimum of jet-lag.
Andrew