How far and the cheapest way to travel between these two?
Gatwick to STN and return?

Gatwick is south of London and Stansted is north of London, so it's
a fairly long trip.

The easiest way to go is to take the National Express coach (bus for
Americans), but it's rather expensive.

If you can carry all your luggage, it is much cheaper and just as fast
to take the train from Gatwick into London Victoria, the tube to
Tottenham Hale, then the train to Stansted. Or there's an even
cheaper route taking the bus from Stratford.

This nice web site explains the options:

http://www.londontoolkit.com/travel/...d_cheapest.htm