First, it is not quite accurate to say humans were once gorillas. It's like saying that you were once your cousin. Humans and gorillas both descended from a common ancestor as did you and your cousin. But it is not like you descended from your cousin.
I do believe in the idea that human beings descended from ape-like creatures over long periods of time. I think that the fossil record shows quite clearly that there have been many different species going from very primitive things that were little different than apes, up through more "advanced" creatures, and eventually to Homo Sapiens Sapiens.
Many people question this because the Bible says God created Adam and Eve in his image. I do not believe that this is meant to be taken literally. I think it just means that God created the world and everything in it (whole universe, for that matter). But the Bible was not a science text and was given to people who were way too primitive to understand evolution. That's why I can believe in both God and evolution.