Western Lowland. I think there are only Western Lowland gorillas in North American zoos. There was 1 Eastern Lowland gorilla in captivity but he/she passed away a few years ago. I don't remember the zoo.
The last Eastern Lowland Gorilla in North American zoos was Mac or M'Kubwa who lived at the Houston Zoo. He was 51 when he was euthanized on May 21, 2004. You can view more info at monkeywire (a great list to join, BTW): [monkeywire] A sad goodbye as sick gorilla is euthanized. You can view a photo of him at the Gorilla Haven website: Crisis in the Congo.