How common are side effects (weight gain/loss, trouble digesting, etc) after gallbladder removal?

Fairly rare. The vast majority of those undergoing cholecystectomy (removal of the gallbladder) do not experience any of the things you mention. Occasionally people have trouble digesting very fatty meals but that is not good for you anyway and can be managed by a moderately low fat diet. Changes in weight are not at all related.
PCS. Some have reported symptoms sometimes known as post cholecystectomy syndrome. The gallbladder stores bile which aids digestion & breakdown of fats. Once that storage function is gone, more bile may travel upwards causing gastritis & esophagitis, or people complain of gas and bloating who may not actively secret bile quickly enough to compensate without storage & have impaired fat breakdown.
Weight changes. Many people complain of weight changes after (or even before) gallbladder surgery. The gallbladder has no metabolically active role in the body. It merely aids in digestion. Typical reasons for weight gain after gallbladder surgery deal with increased caloric intake. If patients feel better, they can eat more, hence may gain weight. Simply, calories in versus calories burned.
Minor but common. One of the more common side effects are loose stools from loss of the gallbladder. Not to worry, this usually resolves in a few weeks. Otoh, many patients have a period of constipation related to narcotics used for pain control.
After GB out... I estimate 15-20% of people have problems of some sort after gb surgery. But - I am 'biased' because I typically see those that have problems, not those that don't.But, gb disease can be fatal - depending on cause. For example gallstones can lead to pancreatitis and this can have bad outcomes...