It cannot be used as a noun. In the sentence "Hey, gorgeous!", gorgeous may SEEM like a noun similar to "Hey, Mary!" but it is not. Actually the sentence is incomplete, but it is a common sentence. The full sentence might be "Hey gorgeous woman, good to see you!" "Hey gorgeous girl, look over here!". We can drop all of the other words as in this case the main thought is "you look good" and the other words are understood. Almost any adjective could replace gorgeous: Hey beautiful, ugly, smelly, shorty, noisy, nosey, etc.

No, you can't, because it's an adjective. In the example sentence you gave, it is being used as an adjective, not a noun.

The word gorgeous is actually an adjective if you want to use gorgeous as a noun you have to use the word gorgeousness. I hope this helps!