Where can you find evidence of Gatsby's devotion/obsession with an ideal, rather than an actual person[Daisy]?The first sentence of this question...

Jay Gatsby is a dreamer.  There are several facets of his
dream.  One is the Horatio Alger success story that involves discipline, hard work,
drive--the rags to riches idea.  This dream was begun in his youth, as his father
reveals, and one that he obviously has attained in his
adulthood.

The second dream was fashioned by Dan Cody.  Dan
Cody on his yacht introduced Gatsby to the lifestyle of the rich, and this lifestyle
becomes the second part of Gatsby's dream:  Cody's yacht "represented all the beauty and
glamor in the world."  Gatsby with his cream colored car and mansion in West Egg has the
kind of lifestyle that Dan Cody represented for him.

Daisy
is the third part.  What Daisy represents to Gatsby is more difficult.  She is the face
that he puts to his dream.   Gatsby is in love with Daisy's life.  In Chapter 8, we
learn that Gatsby falls in love with Daisy and her
house:

Her
porch was bright with the bought luxury of star-shine; the wicker of the settee squeaked
fashionabl as she turned toward him and he kissed her curious and lovely moth.  She had
caught a cold and it made her voice huskier and more charming than ever and Gatsby was
overwhelmingly aware of the  youth and mystery that wealth imprisons and preserves, of
the freshness of many clothes and of Daisy, gleaming like silver, safe and proud above
the hot sturggles of the
poor.

Daisy represents Old
Money, love, and more importantly eternal youth.  Attaining Daisy in the way that he
wants is impossible.  Gatsby wants to turn back the clock to the time when they were
young and in love, and when he appeared to her as a rich officer in the military, and he
could come and go in that glittering house of hers.  What Daisy represents to him is a
missed opportunity of the kind of love and lifestyle that had he been a rich young man
he would have had.  Having the wealth now does not mean anything because all the money
of the world cannot make Daisy a woman who never married and who never had a child.
 Neither can his money now make him a rich young man in his youth, who would have been
Daisy's equal.  This is the dream he is chasing.  It is this dream that cannot be
attained even if Daisy sleeps with Gatsby in the afternoons or leaves Tom and marries
Gatsby.  He cannot have her as she was in her "white girlhood" in
Louisville.