Well, it's not so much a matter of distance, but traffic and limits in the transportation system. The actual distance by road between those two airports is 70km (about 44 miles).

The only direct public transport between London's Heathrow and Gatwick airport are the National Express coaches which shuttle between the two airports.

If your airline ticket arrives at one airport and departs from another your transfer is not included in the flight ticket cost. Even though you may be ticketed through both airports, you will still have to personally transport your luggage between Heathrow and Gatwick.

Allow 3 hours minimum from landing to check-in at the other airport.

If you have more than 8 hours between flights, you may consider taking the Gatwick Express Train to London Victoria, which needs about 30 minutes for the run, for a little sightseeing in London. You then can take the Heathrow Express Train from London Paddington Station (no Joke !!) to Heathrow Airport. This services needs about 15 minutes for the run.

In any case, be prepared to pay quite handsomely for the fares. You may get the feeling you are buying the Train, instead of just a Ticket for the journey.