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.