Just removing of gallbladder does not hamper your ability to digest fat completely. There is slight reduction in fat digestion which is insignificant. Reason of your Being underweight can be very different. You must calculate your BMI. A proper assessment of your nutritional status will help you  to tackle your low weight problem Your problem could be specific nutritional deficiency iron deficiency calcium deficiency protein deficiency or digestive disorders.

There is no reason to avoid fatty meal due to abscence of gall bladder. You can digest fatty meal even without gall bladder. No need to restrict diet. Ofcourse too much fatty meal none can digest even with gall bladder.