By car
Gatwick to Heathrow is about 40 mins
Heathrow to Luton about a hour
Gatwick to Luton a bit of a schlep of over an hour
There is public transport which is a hassle and a big schlep.
EasyJet are very strict on luggage allowance but easy on hand luggage as long as the bag fits in the overhead compartment. They charge for everything. There is no preseat allocation, its first onboard grabs a seat. There are no tickets just online etickets. No food is served on the plane but its roomy as long as you’re not a giraffe
Go to http://www.easyjet.com
All in all they’re not a bad airline as long as you allow for possible delays