I don't have much experience with dog fights.. however. I'm curious if you have a yard or anything or other means of exercising your dog other than the dog park?
I am not against them, at all, I have had very good experiences with them and have a great one by my place that we visit every now and then. However, that is not the case for many. I go with the knowledge that I usually have a good time there with no issues, BUT I also go with the knowledge that there may infact be an a-hole dog there with their owner that I have to deal with.
The first thing I noticed in your story is that you said when you got to the gate the dog was already acting aggressively. That should have been your cue to leave and come back another time if, at all. I am glad the owner was proactive, but that won't always be the case.
My advice would be to find other avenues to exercise, play,. and socialize your dog so that you don't rely solely on the dog park.
Commander Charly Titan (GSD)-15 Jan 2010