In one of the most important thematic passages in the novel, the metaphor of the ladder occurs in Chapter 6:
. . . the blocks of the sidewalk really formed a ladder and mounted to a secret place above the trees--he could climb to it, if he climbed alone, and once there he could suck on the pap of life, gulp down the incomparable milk of wonder.
Jimmy Gatz dreamed his way out of North Dakota. He ran away at seventeen to find the world he wanted, a place of success and wealth, glamour and excitement, beauty and enchantment. In those years, "his heart was in a constant, turbulent riot" as he searched for the amazing destiny he felt certain was his.
His was a self-absorbed quest, although women played a role in his young life: "[Gatsby] knew [women] early and since they spoiled him he became contemptuous of them." Daisy was the first "nice" girl he would know. He had no intention of falling in love with her.
However, when Gatsby and Daisy walk along the sidewalk one autumn evening, he understands he is at a crossroads in his life. The ladder is a metaphor for finding his life as he had always envisioned it, the life he had always chased. He knew he could find it--if he were alone. Instead, he kisses Daisy, knowing that "his unutterable visions" would be "wed" to "her perishable breath." When they kissed, "the incarnation was complete." At that moment, the physical Daisy became Gatsby's romantic dream. His dream was corrupted; his future and his fate were determined.
The significance of the ladder at the end of chapter six with which Gatsby could reach 'a secret place above the trees [...] if he climbed alone,' is that this ladder represents to Gatsby a path to personal peace and a heavenly state.
F. Scott Fitzgerald is indicating that this secret place above the trees is in essence a heaven... or possibly a Garden of Eden to Jay Gatsby. The rest of the line from this part of Chapter 6 continues...
", and once there he could suck on the pap of life, gulp down the incomparable milk of wonder."
This pap of life, milk of wonder represents eating of spiritual food that results in bliss and a Utopian existence whereby Jay Gatsby loves and is loved. It is an existence where his personal dreams come to full realization.
Gatsby must "climb alone" because this is his personal journey that he must work out. In other words no one else is responsible for his happiness but him; his success regarding love, and in essence, his success regarding all areas of his life are dependent upon him making right decisions.