Nick first notices Jordan at Tom and Daisy's house. I think his first reaction is as an observer. He walks into his cousin Daisy's house and what he sees of Jordan is that she is this woman lazily sitting on the opposite side of Daisy when Nick walks in. As the narrator, Nick takes careful time in describing to the reading audience that she doesn't really move, that her body is fully extended at her end of the divan and these are his quiet observations. Nick is a man who reserves judgment and he tells us so in the beginning of chapter one. So, he notices her athletic body and her reclining nature (at least in this circumstance), but doesn't judge her in any way for that. This is their first non-spoken encounter.
After a time, Daisy makes several references or suggestions at Nick's character that maybe Jordan should consider him an eligible bachelor or vice versa, but Jordan maintains a mysterious quiet attitude throughout a long portion of their first encounter together.
In Ch. 1 Nick drives over to East Egg to meet Tom and Daisy. He first observes Miss Jordan Baker and Daisy relaxing on the couch in Tom's house. Nick's first impression of Miss Baker is as follows:
The younger of the two was a stranger to me. She was extended full length at her end of the divan, completely motionless, and with her chin raised a little, as if she were balancing something on it which was quite likely to fall. If she saw me out of the corner of her eyes she gave no hint of it—indeed, I was almost surprised into murmuring an apology for having disturbed her by coming in.
It is obvious that Nick is unnerved by the confident and complacent attitude of the stranger on the couch and he almost apologizes to her. Soon, Daisy mentions to Nick that the girl's name is "Baker," and the nonchalant manner in which Jordan acknowledges the introduction makes Nick respond in the following manner:
Almost any exhibition of complete self-sufficiency draws a stunned tribute from me.
It is obvious that Nick's first reaction is one of complete awe on seeing Miss Jordan Baker for the first time.