My daughter has angelman's syndrome, she is due to have a gastrostomy, is this a common procedure?

Yes. Sometimes the gastrostomy tube surgery is done in conjunction with a fundoplication. If she has reflux then most likely they should be done together. Doing a g-tube alone may promote vomiting and inability to feed your child. Check with your doctor to see if she has been evaluated for reflux.
Yes. Children who are unable or who are unsafe to take feeds by mouth often undergo placement of a gastrostomy tube. This is a common procedure in such children.