Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Gritty is one of those words usually reserved for intense cop dramas with stark lighting and lot of death. However, Mamet's world of real estate is a gritty one in its own right. This is a world where people don't pull their punches and where every action could mean you find yourself out of work:
ROMA: What's the fucking point in any case…? What's the point. I got to argue with you, I got to knock heads with the cops, I'm busting my balls, sell you dirt to fucking deadbeats money in the mattress, I come back you can't even manage to keep the contracts safe. (2.1.214-219)
Probably not the way you want to talk if you're hanging out with your mom… or in school… or babysitting… or, you know, just about anywhere doing just about anything. These guys swear likes it's going out of style, and they use language to cut at each other and break people down in addition to expressing their own frustrations. Their lives are full of tension and a heaping pile of darkness, and their words reflect it.
Mamet brings us into a dark and gray and gritty world. If you're in need of a little Technicolor pick-me-up, here's the bright and shining trailer for The Wizard of Oz.