What evidence suggests Gatsby is devoted to an ideal rather than an actual person?

In Chapter 6, Gatsby reveals that he wishes for Daisy to erase the years of her life during which they were apart and transport back to the last day they had been together in Louisville:

He wanted nothing less of Daisy than that she should go to Tom and say: “I never loved you.” After she had obliterated four years with that sentence they could decide upon the more practical measures to be taken. One of them was that, after she was free, they were to go back to Louisville and be married from her house — just as if it were five years ago.

Gatsby is desperate for Daisy to tell Tom that she never loved him. If Gatsby could see Daisy as an actual person, he would understand that in the five years they were separated, she had her own life and time passed for her. During that time, she married Tom Buchanan and had a child by him. If Gatsby cared about Daisy as a person, he would not ask her to delete those years from her life. He would not want to go back in time. He would accept that time had passed and that Daisy had done things during the years he was not with her, and he would be able to move on from that point forward.

The idea of going back in time shows that Gatsby cherishes the feeling he had when they were first together, not Daisy as a person. In Chapter six, Nick tells Gatsby, “You can’t repeat the past.” However, Gatsby vehemently disagrees, showing that it's not Daisy he wants, but the feeling he had when he was first with Daisy.

Throughout the novel, Fitzgerald uses the green light at the end of the Buchanans' dock to symbolize Gatsby's desire to possess Daisy. At the end of Chapter 1, Nick arrives home one evening to see Gatsby for the first time. Gatsby is standing on his own dock, reaching out for the green light, trembling. In a way, this introduction to the title character shows that the essence of Gatsby is his desires for an idea, a concept, symbolized by the green light.

At the end of the novel, Nick's final words to the reader explain the green light:

Gatsby believed in the green light, the orgastic future that year by year recedes before us. It eluded us then, but that’s no matter—tomorrow we will run faster, stretch out our arms farther. . . . And then one fine morning—

So we beat on, boats against the current, borne back ceaselessly into the past.

These words show that Gatsby's dream was never attainable. We know he got Daisy, so Nick's final words show that there was something more that Gatsby desired. Something out of reach that he would never have obtained under any circumstances. Gatsby's life was about reaching for that which he desired, not about getting it.