I actually had the same thing last week and I am 36. I asked my doctor the same thing. In fact, the gallbladder is a reservoir for bile and salts. The liver is what breaks down the fats. The gallbladder serves as a storage area for bile that is secreted from your liver. When your liver doesn't break down the fats, it can form gallstones.
There are 2 types of gallstones, ones that form from fat and ones that form from billirubin and bile. After my surgery, my doctor recommended a gallbladder diet, consisting of low fat high fiber foods. But, it is not life or death. It will be easier on my digestive tract.
Did you have an ERCP as well? If not, you may still have bile in your ducts that can cause more pain in the future.
As far as the weight gain issue, I do not know of anyone that has gained a lot after the surgery. And my best friend had the surgery 5 years ago. I would not panic. Just eat sensibly and keep active. But to be sure, tell your doctor your concerns.
I wish you a very speedy recovery!