After a surgery of removing a fibroma, should i expect weight gain?

No: If anything, you may lose weight as you recover from the operation in the hospital. Sitting in a hospital bed can reduce your muscle mass, and the recovery period can sometimes alter your nutrition status. Both of these can lead to mild weight loss. It's important to stay active and eat healthy immediately following the operation and once you go home.

No: Only if you eat a lot.

Not really: You shouldn't gain weight after surgery. If you lie around in bed or on the couch and don't do any activity you must limit your daily caloric intake to your BMR (basal metabolic rate) to maintain your current weight. If the calories you eat are equal to or less than your BMR you will not gain weight. You will only gain if you eat more than your bmr. http://www.bmi-calculator.net/bmr-calculator/.