One of my best friends had her gallbladder removed last summer. She was studying abroad in Madrid and she would get violently ill after every meal. Stomach pain, nausea, vomitting. She thought that it might of been that she wasn't use to the different foods there so she kept on eating and drinking lots of alcohol. I'm sure the later of the two didn't help at all.
When she got back to the US, her father (who is a doctor) saw how much weight she lost which was about 20 pounds and after hearing her symptoms, he immediately rushed her to the hospital. She had a bunch of gallstones and the gallbladder was so damaged that it had to come out ASAP.
Following surgery, you really need to make sure that you eat healthy. The gallbladder is there to hold bile which absorbs fat. Although the liver will take over the gallbladder's function, it's doesn't do as good of a job so many people will gain weight after surgery. My friend put back on all the weight she lost and then some. Even now that she exercises 4 times a week and eats healthy, she still has a really difficult time losing and maintaining her weight.
Make sure to ask your doctor for a list of foods that you should that will help keep the weight off.
I hope everything goes well with your surgery!
ETA:
The doctors think her gallbladder problems were caused by a diet pill she was taking for about a year. She was only 22 at the time of removal and most young people don't have gallbladder problems. I can't think of the name of the diet pill she was taking but I'll ask her and let everyone know. Reseachers have been finding that people have had a lot of health problems from this particular diet pill brand.