what are the distances between these two airports, and is there easy transportation from heathrow to gatwick? I am try to save money on airfare from los angeles to madrid, and if I use easyjet which leaves from gatwick I can save several hundred dollars. thanks

You will want to take the Speedlink bus. It cost 17 pounds one way. Allow plenty of time. I had over 4 hours in May and missed my Gatwick flight.

valid point grasshopper- can be nervey connection to next flight
here's the link you need to read more

http://www.speedlink.co.uk/speedlink_page.html

Without knowing when you will be flying, I will again suggest the possible better solution.
British Airways has 'new' pricing structure for inter-Europe flights. They compete very well with the discount airlines now. The best part is that you could fly from Heathrow, so even if they are maybe 15BP more, it's still a better deal, after you consider the cost of transfers and the pain in the butt of actually doing a little tour of southern London.
DO NOT try to price the ticket from US to Madrid on BA, as that will return a higher fare. Just go to www.ba.com, change the country of residence on top of the home page, right corner, to United Kingdom, and then proceed to make a reservation. Just out of curiosity, I did price a R/T from LHR to MAD in mid-August and it came back around 66 BP including taxes.
Good Luck, and have a great trip!

It's also possible to take a National Express coach (www.nationalexpress.com) for 15 GBP. Perhaps another Fodorite can tell you, from personal experience, which is better, Speedlink or National Express. As others have said, allowing plenty of time is vital. Frommer's Budget Travel Today says that five hours is minimum and six hours is better.

We took the Gatwick Express form Gatwick to Victoria Station. 30 minutes and GBP11. I don't know why you can't take the tube from Heathrow to Victoria Station.
John

Yes, you can take the tube.
Tube from LHR to South Kensington. Change to District/Circle line to Victoria.(prob 50 mins, including change time @ S Ken)

Hump bags up stairs to Gatwick train (and you'll save a bit taking the Connex train,rather than the Gatwick Express, if Connex is running by the time you get here, as Connex has just been fired from its franchise). 45 mins after departure, arrive LGW. Say 2 hrs from LHR to LGW, if the Circle Line behaves itself (which is about as likely as the M25 between LHR and LGW being clear)

After an overnight flight, with all the fun of Gatwick and an Easyjet flight ahead of you, you'll begin to start thinking really nice thoughts about that Iberia direct flight to Madrid. Especially if you've bags, and it's a warm summer day (see everywhere on this board about the tube on hot days)

Take AAF's advice, or budget your time so you can get the direct bus between the two airports. Though only 40 miles apart, the connecting road is Europe's busiest. All day.

thanks for all your advice.I think i'll give up the idea of easyjet, as sounds too complicated.

We did speedlink last time and it was easy. I don't remember the cost. It took about an hour to go from Heathrow to Gatwick. I probably wouldn't do it just for cost. We did it because we couldn't fly direct anyway.