The sentence is “You look pretty tonight”. I believe that pretty would be an adjective as it’s referring to “You”.
However in the sentence “You look pretty gorgeous tonight” I believe that pretty would be an adverb.
In the first sentence is pretty saying how “You” looks though, meaning it would be an adverb?
In the sentence,
You look pretty tonight
Pretty serves as an adjective and “look” serves more of the function of linking verb than action verb.
In the second sentence,
You look pretty gorgeous tonight.
Pretty does indeed serve as an adverb modifying the adjective gorgeous.
You have made some interesting observations here. The verb “look” is a linking verb here like “to be” instead of describing an action.