A - . . . he stretched out his arms toward the dark water in a curious way, and, far as I was from him, I could have sworn he was trembling. Involuntarily I glanced seaward — and distinguished nothing except a single green light, minute and far away, that might have been the end of a dock. When I looked once more for Gatsby he had vanished, and I was alone again in the unquiet darkness.
Foreshadowing uses specific words or phrases to hint at something that will occur later on in the story.
I hope this helps! :)