Having index.html in the gallery folder has been mentioned, but noone has mentioned this - don't name your galleries index001.html, index002.html, index003.html etc.
Instead only use odd or even numbers i.e. index001.html, index003.html, index005.html etc - you'll get a lot of surfers typing in index002.html just to see if it exists.
Just make sure your 404 redirects are set up.
