The most grotesque and fantastic conceits haunted him in his bed at night.
“Grotesque” and “fantastic” are both words used to describe the Valley of Ashes in Chapter 2:
This is a valley of ashes — a fantastic farm where ashes grow like wheat into ridges and hills and grotesque gardens; where ashes take the forms of houses and chimneys and rising smoke and, finally, with a transcendent effort, of men who move dimly and already crumbling through the powdery air.
The repetition here signifies Gatsby’s current place of humble beginnings. The fact that they “haunt” him illustrates his hyper-awareness of his social position and desperation to escape.
However, his conceits, or dreams of self-improvement, are constantly on his mind, even so much as to make him lose sleep. He continues to get lost in his imagination with obsessive dreams of grandeur.
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