GBE stands for Grammar By Example. It is a technique, integrated in XMLBooster, which builds a reasonable meta-definition based on a number of sample XML messages.
GBE builds a meta-definition which matches what a programmer might have coded. It uses simple but efficient heuristics to recognize integers, reals, booleans and even enumerations.
Of course, GBE is limited in the sense that it cannot detect a degree of liberty unless it has been tested in the sample. For instance, XMLBooster will not mark an attribute as optional unless it has been omitted in at least one instance of its enclosing element.
All in all, GBE is a great tool, as it reduces the time to develop meta-definition dramatically, and in the way it assists the user in better understanding the XMLBooster formalism, by showing in examples how a meta-definition can be built to match sample XML messages.