I'd like to know if "blankly" in the following excerpt from the Chapter Five of The Great Gatsby means "with confusion" in this instance (his mind is not on the conversation, it's wandering elsewhere. Thus, he isn't really listening, but he is half listening), or rather "without expression," in relation to the adverbs "hollowly", "vaguely", "miserably" (which reflect the failure of his dream; they suggest it is as if he already lacks life):
“Is everything all right?” he asked immediately.
“The grass looks fine, if that’s what you mean.”
“What grass?” he inquired blankly. “Oh, the grass in the yard.” He looked out the window at it, but, judging from his expression, I don’t believe he saw a thing.
In this context, "blankly" describes the blank look on Gatsby's face. He is filled with anticipation and uncertainty about finally meeting Daisy. His mind is not on Nick or the grass or the flowers. He has a blank look on his face because his thoughts are focused on the prospect of meeting Daisy. His thoughts are not on the present moment. Rather, they are on the immediate future.
"Blankly," in this sentence and context, means "without expression" but you could argue that it also means "with confusion" because Gatsby might be confused by his feelings of hesitation (at the prospect of finally meeting up with Daisy) after years of planning his and her reunion.
To be more specific, Gatsby's blank look shows that while he is physically present and having a conversation with Nick, his mind is not focused on those present things; his mind is focused on the upcoming meeting with Daisy and all the hopes and doubts that go with it. Therefore, the term "hollowly" is used in a similar way because Gatsby is physically present but mentally (inside), he is somewhere else. His face was blank ("without expression") because his mind was on other things. He had "vacant" eyes which is the same as saying he stared blankly:
Gatsby looked with vacant eyes through a copy of Clay’s ECONOMICS, starting at the Finnish tread that shook the kitchen floor, and peering toward the bleared windows from time to time as if a series of invisible but alarming happenings were taking place outside.
His mind is in a state of frantic anticipation about the upcoming meeting and on the past five years of his quest to win Daisy back. Quite literally, his mind is on the past and future. Since his mind is on things other than the present, his expression appears blank and vacant. "Absentminded" is a bit more accurate than "without expression" or "with confusion" because "absentminded" connotes a sense of being lost in thought and unaware or unconcerned with the immediate, present surroundings.