I understand that you are positive you saw a gorilla, but this is a memory from almost forty years ago, when presumably you were a child. Memories are not reliable.Were you at the zoo in the 80 s ?
There was one that definitely looked like a gorilla.
Not a chimpanzee.
More than twice the height and weight of a human man.
Not a silverback.
Western gorilla. All black. Chimpanzees were in a block of pens and a quarter it's size. Not even close. I have never seen any animal that big before or since. It was enormous.
Tetrapod, who posted earlier in the thread, used to work at the Perth Zoo.
The studbook for gorillas is available online - you can check it for yourself - and there was never any gorilla kept at Perth Zoo. Even though I already knew that I just went through it anyway just to be sure.
What you saw was almost certainly one of their chimpanzees.