As the name suggests, a GY is a year in which you take a break from everyday life, usually for one school year or twelve months, and take the opportunity to start another activity, usually in another country. This gap year can occur at various times in life, from youth to old age.
Either way, GY is a very broad concept that goes beyond any attempt at definition, so it all depends on what you want to do.
Information about the event is available here.