Someone has published videos on YouTube that guide you through all of the gates visually - first part is this link
http://www.youtube.com/watch?v=EMO24rVYIIw
There are 7 videos in total on his account for all 135 gates. Even including some reserve gates (there are actually more than 135 and some don't count)
Btw actually almost everybody has that issue missing just one security gate (I had that too) - the reason is that there is most likely a glitch that one security gate you found has not been correctly counted and you have to check all of them again. When you find it you may be surprised that it has not been counted already. When checking all gates again I found my last gate at the lighthouse entrance and that was so bloody obvious that I could not have missed it.
