What's the difference between bacon and gammon?
I did my internship for culinary school in London, and if I recall correctly from what I saw in the markets there, what you call Gammon would be equivalent to our Ham - both coming from the pig's rear leg.

What we call bacon is what you'd call "streaky" or "streaky bacon" which is made from the pork belly.

Streaky would be the best substitute in recipes calling for bacon.