Gatsby is seen for the first time in the narrative in a symbolic pose, standing on his balcony in the night with arms outstretched and open, reaching for something that is not there.
This image reinforces the dreamlike quality of this hero on a quest to attain his dream. (eNotes)
This vision of Gatsby immediately suggests his character's symbolic significance or symbolic aspect. As the story progresses details and accounts of Gatsby's life render him as a realistic person, with a history, a heart, and an income.
Details of his experiences with Dan Cody and his time abroad serve to create a reality and fullness to his character along with the more present details of his personality that Nick encounters in conversation with Gatsby and describes in the narrative.
Despite the growing friendship between Nick and Gatsby and the emergence of realistic details of Gatsby's life and character, Gatsby remains set apart from the rest of the characters. This is partly due to his great wealth and partly due to the insulation that his dream provides.
Gatsby is described as possessing a “heightened sensitivity to the promises of life” and as being dedicated to "the following of a grail”.
These qualities shield or buffer Gatsby from other characters and from reality, maintaining his potency as a symbolic figure.
Descriptions of Gatsby's exceptional gifts for dreaming and his related, special type of innocence effectively reinforce Gatsby's significance as a symbol. We can see that here, as Gatsby is described as an "epitome", a crystallization of qualities setting him apart from the norm:
[Gatsby had] an extraordinary gift for hope, a romantic readiness such as I have never found in any other person and which it is not likely I shall ever find again. (The Great Gatsby)
More than other characters in the novel, Gatsby is closely associated with a set of ideas. His passions and ideals are simplified and repeated in such a way that his character becomes convincingly symbolic; the ideas he stands for clear and present in the life of the narrative.
Gatsby believed in the green light, the orgastic future that year by year recedes before us.… Tomorrow we will run faster, stretch out our arms farther.… And one fine morning— (The Great Gatsby)
Where Tom, Daisy, Jordan and Nick are sometimes objectified, they remain subjects, capable of action and of change. Gatsby is not capable of change. Rather, as he is presented and described, Gatsby is a finished product, made by the dream that he espouses and ultimately destroyed by it.
This static or finished quality helps to support the notion that Gatsby is a symbolic figure. However, the details of his dress, his demeanor and his dreams are all so vividly presented that he is also a realistic character.