This is some crazy surrealism, in our Gatsby sentence today. (In the Gatsby project, we look, each day, at a single isolated sentence from "The Great Gatsby.") But let's settle down and understand what we are looking at. It's Gatsby's visual perspective. "Out of the corner of his eye" cues us that it's subjective and distorted. He's looking at the sidewalk and the perception is that it looks like a ladder. We're told that it really is a ladder, but we know that's not true. It really is a sidewalk, but to Gatsby subjectively, it's a ladder. Gatsby imagines himself climbing somewhere. Mounting. It's a secret place.
"... he could climb to it, if he climbed alone..." — I want to say that the illusion only remains intact if Gatsby is alone. And the vision, if he can hold himself within it, is of climbing that ladder up through the trees and then finally coming alive.
It's a vision of being born, just by walking down that sidewalk. And then he could be alive. He'd suck on the maternal breast, the pap of life. He wants to gulp it down — that incomparable milk of wonder.
He's just a man on a sidewalk. There's no ladder. He's already born and living in real life. But somehow it seems that real life is out there, unreachable. But if only you could get there, above the trees, you would drink it down. Incomparable! Wonderful!