A GTY Cabin is a Guarantee. It means that you give up the ability to select your cabin, but you are guaranteed to have a cabin in the category you paid for or better.
For instance, a lot of people book a Cat 12 cabin, but can get upgraded to a Cat 10 for free. Upgrades are totally random. The only guaranteed upgrade is booking 4 in a cat 12 and you would at least be upgraded to a Cat 11 as Cat 12s can't hold 4 passengers.
Hope that helps.