Hi Sherri,
I saw someone already explained GTY (you are guarenteed to get the category you paid for or better).
In terms of Plexiglass or white wall.... there are 3 types of verandas, and all 3 types are different categories (and all 3 categories are different prices). One category is a Navagator's veranda, which I think is what you have. A solid white wall with a porthole cut out of it. I believe that is the least expensive veranda category (5C)
Then, there is the solid white wall (half-wall) on your veranda. So, if you're sitting in a chair on your veranda, you won't be able to see much unless you look up (or you're VERY VERY VERY long wasted.... hahaha

). With the solid white wall, if you want to see the water or land, you pretty much have to just stand out on your veranda. That is the 2nd least expensive veranda category (5B)
Then, there is the veranda with the clear plexiglass, so you can sit out on your vernada in a chair, and you can still see out because the entire wall (except the railing) on your veranda is clear. This is the 3rd least expensive veranda category (5A)... that is the category we have a GTY for.
Then, there are all the category 4's, which I believe are family staterooms w/ verandas... and I "believe" the categories are the same as the #5 categories in terms of verandas, but the category 4's just have a bit more square footage in them to accomodate 5 people, for families of 5, which is why they are more expensive veranda rooms than the category 5.
So, when I asked my question, what I'm concerned about is that Disney would upgrade us to like a Category 4C or 4B (because GTY sometimes you are upgraded if they do not have any rooms open up in the GTY category you've purchased). Well, a category 4 WOULD be an upgrade because we'd have a room with a bit more square footage, but if they put us in a 4C, or 4B, that'd mean we would not have the clear plexiglass on our veranda, but a bit more square footage in our room (as well as probably on the 8th deck, so a bit higher up on the ship). But, the reason we paid for the 5A GTY is to guarentee the clear plexiglass so just like another poster said, we can just sit on our veranda and see out, so for us, it'd be a bummer to get an upgrade to a cat 4C or 4B because we don't care about the square footage in the room or being up on deck 8. We just want the clear plexiglass veranda.
We have small kids, and they love being able to just come out on the veranda and see out (we've had that type of veranda before, and love it). Also, my DH and I loved just getting a drink, sitting outside on our veranda and enjoying the scenery while sitting. For our family, totally worth the cost of the plexiglass veranda.
Anyway, hope that all makes sense!
Kathy