I am an OB/GYN biller and use the GB modifier often. There are many times we are seeing a pregnant patient who then decides, for whatever reason (moving, etc) to switch to another office. All of her antepart visits are suppose to be included in the delivery. Now that we will no longer be delivering her baby, all the antepartum visits are billable individually. It is in these cases that I use the GB modifier appendend to all the antepartum visits. It still sometimes requires a call to the insurance company, but for the most part they are now aware these visits are not included in the global maternity package. I don't have documentation that states it is for "rebilling" a procedure not included in the global maternity package.
Hope this helps!