Gatsby spends his adult life recreating his persona, acquiring wealth, and moving closer to Daisy. She had become his "holy grail" (the American Dream) and he stopped at nothing (including criminal involvement) to put himself in a position where/when he and Daisy could meet again and hopefully fall back in love. Gatsby is trying to relive the past in the present. Therefore, he is attempting to bypass time itself.
In "If you were coming in the fall," the speaker expresses a similar sentiment. Time and space separate the speaker from herself and her significant other. In spite of this (like Gatsby), the speaker is willing to wait. The increments the speaker would wait increase in each stanza from the summer, to a year, to centuries, to a lifetime and eternity.
But just as Gatsby was never certain he and Daisy would be together again, the speaker in the poem is also unsure.
But now, all ignorant of the length
Of time's uncertain wing,
It goads me, like the goblin bee,
That will not state its sting.
The speaker doesn't know when she will be reunited with her lover. The uncertainty, whether it will happen or not, is a tease always prolonging the satisfaction of a conclusion. Gatsby waits five years. But even when he and Daisy are reunited, he is optimistic but still unsure that Daisy will leave Tom for him. Gatsby was also goaded by this lack of closure, the inability to completely bridge the past and the present.