How invasive is surgery or for a gastrointestinal fistula?

Invasive. It is major abdominal surgery. Generally it is done laparoscopically or as an open procedure, and can be very invasive.
It varies. A fistula is a communication between the bowel and another surface, can be skin or another organ. It really depends on the underlying disease process. They are often seen in crohn's diesease, cancer, post-surgical complications and many states in which the patient is immunocompromised and sick and/or on steroid which can make any surgery more complicated.

Related Questions

Is surgery always necessary to remove a gastrointestinal fistula?

Not Always. Gastrointestinal fistulas are treated according to the organs involved and the output of the fistula. Fistulas between the colon or small intestine and colon can be left alone as long as they are not causing infection or malabsorption. Fistulas from the intestine to the skin are classified as high output (>500 ml/day) or low output (<500ml/day). Low output fistulas generally close without surgery.
Almost always. Most fistulas cause major metabolic problems and are a threat to your health and safety. They are also a risk and cause for cancer if left untreated.

Is gastrointestinal fistula a complication of abdominal surgery?

Can be. Gastrointestinal fistulas can be a complication of intrabdominal surgery. They can also be due to other causes such as inflammatory bowel disease, cancer, radiation, etc.
Yes. This is an uncommon complication, caused when the intestine leaks into the abdomen following surgery. The intestinal contents then leak through the wound to drain out of the body.