No dragons, warriors, or magic spells here—Glengarry Glen Ross is all about real-world problems and real people who are shaped by that world. That said, though, there are some great debates amongst theater folk as to whether or not Mamet is a true realist. For every actor and critic who thinks that Mamet writes the way people talk, there are ten others willing to say that his dialogue is highly stylized and unrealistic. Here's a blurb about Mamet's American Buffalo that many could easily apply to Glengarry Glen Ross:
The language was realistic in the sense that it sounded just as the speech of these rough types should sound. But it is not realism. Mamet creates here and in his other dramas a staccato, rhythmic, obscenity-ridden dialogue that is artistic and arresting, an instantly-recognizable Mamet trademark. (Source)
Regardless of whether or not the play is prototypical realism doesn't mean that it easily fits into any other genre—Mamet's drama adheres to the rules of realism, and it's arguably the best fit. Well, beyond drama, of course. Drama, in its truest sense, is literature written to be performed, and Glengarry Glen Ross definitely fits the bill.