I think that one particular area of comparison between both works would be shared understanding of what it means to be a part of the world. The idea of maturation, in terms of understanding the world and the individual's place within it, is central to both works. Selina must come to the awareness of the pain which is intrinsic to the world. Her own sense of identity is forged through this pain. She understands this from personal and external conditions. From the racism of the outside world to the struggle she faces within her own life in the form of her mother, Selina understands that identity involves struggle, that the subjective must often times come in conflict with the external. Certainly, Fitzgerald emphasizes this in his work. Nick comes to understand that his own moral and ethical code collides with the "destructive" nature of the Toms, Daisys, and Jordans of the world. Gatsby understands that his own condition of being is one where external dreams collide with external reality. In both works, individuals arrive to a sense of understanding through navigating and better understanding an external world that might not affirm their own sense of being and identity.
A significant difference would have to be the narrative voice. Marshall's work focuses on a young woman of color who faces economic and social challenges. This has a profound impact on the work's meaning and its overall statement. Discussions of race, class, and gender are a part of its narrative focus. While Fitzgerald's work might touch on such contexts, they do not drive The Great Gatsby. The issues upon Fitzgerald chooses to focus are fundamentally different than what is featured in Brown Girl, Brownstones. This would have to be a significant point of departure in both works, representing a significant difference between both works.