I had mine out at age 27. I did not have any of the common risk factors, other than being female. (I wasn't fat, I hadn't had any children, and I was not age 40+.)
I did have gallstone pancreatitis, which caused by gallstones that inflame the pancreas. It was very painful and I had had gallstone flair ups for months, if not year(s), before I finally was sick enough to go to the ER and properly diagnosed. During that time before the operation, I certainly lost weight because I couldn't eat much. That lost weight I eventually gained back, I suppose, because I was eating normally.
I was cautioned to avoid fatty foods for a while, which I did for maybe a month or two, but now I have no side effects whatsoever and I can eat whatever crazy garbage I want to now. But in terms of permanent weight gain or excessive weight gain after surgery - not at all.