I personally feel that the 25 dollar bus fare from Gatwick to Heathrow is a big rip off. If you do not have luggage and six hours, you can buy a One day travel card all zones which is good for travel in Rail Tube and bus in entire London costs around 7 dollars. (You might have to buy a one way ticket to east croydon)Use Thameslink not the Gatwick express to Victoria and then tube to Heathrow. You will save all the money as you wont be shopping