A 12gty means that you will sail in nothing lower than a cat 12 (of course, there is nothing lower....but a 6 gty means nothing lower than a 6). We have booked the 12 gty a couple of times and never sailed in a 12. We have always had at least an 11.
As to seasickness, the lower you are on the ship and the more centered you are, the less motion on the ship. SO, on deck 1, you are as low as you can get. You might get lucky and be upgraded to a 9 (with a porthole). More likely, you will sail in an 11. This is the same as a 12, only different location on the ship. We've been in an 11 several times--they are just fine.
So, what you lose in booking gty is the ability to choose a cabin. What you gain is a better chance for an upgrade. However, if you will not be happy in a 12, do not book a 12 gty.
Yes, the 12s are under the clubs, and I've heard reports about a noise problem at night. Some people travel with a white noise machine. I'd just wait and see where you are finally located. If there is a problem with your cabin after you board, talk to guest services. They won't move you with a gty prior to boarding just because you don't like what you got (that's the downside to the gty), but if there is a problem after you are actually in the cabin, they will take care of it if they can't fix it.
