How does the geography of the novel dictate its themes of characters ? (What role does setting play in The Great Gatsby?)The Great Gatsby by F....

First, I suggest that geography and setting don't dictate theme and characterization as much as reflect them, although one particular detail is central to character and theme.

That detail is that Nick is from the Midwest.  His values and ideas interpret the action for the reader.  He is opinionated and makes value judgments about the characters.  For instance, his view of Tom is certainly very much Midwestern.  He is an outsider looking in at Tom's old money, lifestyle, attitudes, etc.  The characterization of Tom certainly might be dictated by Nick's being from the Midwest.  And, by the way, of course, what a character knows, his attitudes, etc., is part of a work's setting.

That said, the geography and setting in the work certainly reflect characterization and theme.  A primary example is the setting and geography of the two Eggs.  Though within sight of each other, they certainly represent two separate "worlds," figuratively speaking.  This highlights the conflict between different segments of American society, generally, and the conflict between Gatsby and Tom, specifically.  It also reflects the theme of the American dream.  The Valley of Ashes, of course, reflects the barren nature of the Wilsons' lives, as the location of the novel--the East--contrasts with both Gatsby's and Nick's upbringings.

Again, the geography and setting reflect theme and characterization in the novel, but it might be a bit of a stretch to suggest that they actually dictate theme and characterization.