The answer to this question comes in the closing paragraphs of Chapter Six. Gatsby is just about to kiss Daisy but, as she approaches him, he has an important realization:
He knew that when he kissed this girl, and forever wed his unutterable visions to her perishable breath, his mind would never romp again like the mind of God.
In other words, when Gatsby kisses Daisy, all of his dreams of their reunion will finally come true. This is momentous: remember that Gatsby has spent most of his adult life pursuing Daisy. He has become rich, powerful, and successful only because he wants to be with her again.
So, for Gatsby, these "unutterable visions" are the dreams he has long held of winning her back. Once he kisses her, these dreams will become a reality, wed to her "perishable breath," which means that they will become real and tangible. They will take on a physical form, just like a person's breath.
In the next line, there is a comparison between God and Gatsby himself. Arguably, Gatsby is just like God because he is the creator of his destiny. He has created wealth and power, just like a god, for the purposes of being with Daisy.
Once he kisses her, however, he will no longer have to invent new methods of winning her back because he will have achieved this goal. So, his mind will never "romp" again because it won't have to. Daisy will be his, just as he has always dreamed.
Kissing Daisy, therefore, means Gatsby must give up his elaborate visions to win this girl back. In one sense, he must give up his very being; he must give up being Jay Gatsby.
The kiss between Gatsby and Daisy five years before is described in biblical, even metaphysical terms. We are told that Gatsby knows that when he kisses Daisy, and "forever wed[s] his unutterable visions to her perishable breath, his mind would never romp again like the mind of God. . . . Then he kisse[s] her. At his lips' touch she blossom[s] for him like a flower and the incarnation [is] complete."
There is a sense that Gatsby makes some sort of sacrifice to be in love with Daisy. His dreams are so great that they are unearthly, but Daisy is grounded in the real world, like a flower. Though she is lovely, she will turn his mind away from being a "son of God" who "sprang from his Platonic conception of himself." It seems that, by falling for Daisy, Gatsby has placed his trust and hope in the wrong thing.