Every wedding that I've photographed at the GBP has paid for the lowest option of bar (beer & margaritas) however a full bar has been served. If it's REALLY important to you, then perhaps you don't want to risk it. However I haven't heard from any of the most recent brides anything different.
nic, here is my photo tour of the property: http://faberphotogra...ort-photo-tour/
It also has a link for the menus of the restaurants near the section about the Gran Tortuga.