Looking at the Gatwick Express site, it seems that teh earliest train leaves
Victoria Staion at 5AM and takes 30 minutes so it arrives outside the
airport at 0530 if it is on time.
That is less than an hour before departure plus walking time to the EasyJet
check-in counter, so it may be cutting it too close.
The journey planner gave a very ugly itinerary from Stamford Brook station
area requiring 4 buses and over 2 1/2 hours travel time.
What other alternatives other than a taxi will arrive between 0430 and 5AM?