In The Great Gatsby, how are "the frosted wedding cake of the ceiling" and "a long white cake of apartment houses" significant?
I like Susan's answer, and I would like to add a couple of other possibilities that explain F. Scott Fitzgerald's development of a white cake motif in The Great Gatsby. It is no accident that both of the passages you cite involve a white cake; in addition to wealth and opulence, as Susan mentions, white cakes are typically associated with weddings. This is especially noteworthy because this motif recalls Gatsby's desire to have an idyllic life with Daisy that will, in his mind, ultimately culminate in Daisy divorcing Tom and running away with Gatsby.
It could also be argued that Fitzgerald's emphasis on white cake imagery in these two passages represents Gatsby's naive world view. Gatsby has a romanticized notion of how his interactions with Daisy should go, and his expectations are damaged when Daisy acknowledges that she loved Tom at one point in their marriage. Gatsby's hopes and dreams, the sole reason he built up his fabulous wealth, are hurt by Daisy's revelation. Thus, the wedding cake imagery could also refer to the naivety of Gatsby.
Both of these passages contribute to Fitzgerald's development of white as a color motif in the novel. Those things which are white are associated with wealth and privilege. In the Buchanans' living room, in addition to the white ceiling (wedding cakes are white) are white French windows and curtains that are described figuratively as "pale flags." Daisy and Jordan both are dressed in white. Nick learns later that when she lived in Louisville, Daisy dressed in white and drove a white roadster. The apartment Tom keeps in the city is located among many white apartment buildings. They are located not far from Central Park, which was, and still is, a very exclusive and expensive place to live. The "white," expensive worlds of the Buchanans stand in stark contrast to the gray poverty of the Valley of Ashes.