Just going to jump in here and back up what everyone else is saying about Kumbuka. I visit London regularly (when possible!), and always seek out some time with the gorillas. When Kumbuka was still new at London, yes he would bang the glass and display, but no more than any other male, newly arrived with a new family and situation, and more than a little put out by the closeness of the public at London. When Alika was born, he was initially very protective, as would be expected with any new dad. Once he settled in, the stand off barriers and camouflage decals added to the glass viewing, and especially once his offspring started to get bigger, he really calmed down and became very chilled. Even the noisy public didn't really seem to bother him any more.
I admit I don't know gorillas as much as some members on here, but I would never have described Kumbuka as aggressive.
Pretty good encapsulation of the story I think...