Weight gain after gallbladder surgery is a common complaint and may have a variety of causes. After the gallbladder has been removed, many people are able to eat fatty foods again without risking the negative side effects commonly associated with gallbladder disease. Weight gain may be caused by of the difference in the way the body metabolizes fats after the organ is removed. Medications, stress, and a lack of sleep may also contribute to the weight gain that often follows this type of surgery.

 Dietary changes are believed to be the primary reason why people gain weight after gallbladder surgery. When the gallbladder stops functioning properly, it may become difficult to eat fatty or greasy foods without experiencing pain, bloating, or other types of digestive disturbances. After the organ is removed, these symptoms often disappear, allowing the patient to eat foods that were off limits before the procedure. It can be easy to eat too many of these unhealthy foods once they are well tolerated by the body, resulting in weight gain.
Without the gallbladder, the body must learn how to metabolize fats in a different way, sometimes leading to weight gain. Bile is normally stored inside the gallbladder, but when this organ is removed, it is secreted into the small intestine. If there is not enough present, it can cause weight gain. Eating several small meals throughout the day instead of three larger meals may help to

Source: http://www.wisegeekhealth.com/what-causes-weight-gain-after-gallbladder-surgery.htm