quote from lespaul:
Do you really want the verandah? Get the Cat 5 guarantee, then you will definitely get the verandah. You will get whatever stateroom is left on deck 5, but it could be a great room.
endquote
Actually, a Category 5 cabin would be on Deck 7. If you have a Cat 5 GTY, you're assured of getting a Category 5 cabin, at a minimum and perhaps being upgraded to a higher category. The GTY just means that there are more requests for that category than there are available cabins. For example, if there are 20 cabins available and 24 requests, then 4 people will get higher categories.
I'm just guessing as to why there aren't any Category 6 GTYs available, but I think they have many more requests for cat 6 than they can accomodate. With the above example, they have 20 available and 32 requests, and from past experience, they know they'll have to upgrade all new requests after they get 25, so they stop taking new requests after that.
I also wouldn't think that you would get the worst cabin in Category 5. You would just get an unassigned cabin and it just means that no one requested that cabin, not that there's something wrong with it.
Like the other posters, I think it's your best bet for getting a cabin with a Veranda.