Jordan and Nick have a relationship built largely upon convenience. When Jordan feels the need for an escort or a companion for an activity, Nick is one of the collection (probably) of persons she can ring up to fill the role. Nick finds Jordan physically attractive but realizes that she is not a good person. Theirs is physically more than a casual relationship for awhile, but then they evolve to a slightly charged friendship.
Jay Gatsby is infatuated with Daisy and engages in fantasies built upon schemes that would allow him to win her away from her husband. Daisy realizes how Gatsby feels about her and likes the fact that he desires her, but she never intends to actually leave the financial security she has with Tom. Gatsby goes to great lengths to demonstrate his feelings for Daisy and to hide the truth about his past and present from her. The relationship never has any chance of surviving reality but it's a great basis for dreaming for them both from their individual viewpoints.