OK I'm gonna go with the urban dictionary on this one
http://www.urbandictionary.com/define.php?term=gritty
gritty
A type of realism, usually invoked by films and documentary. Strangely
enough, "gritty realism" is only perceptible to media and film critics and the term is hardly ever used by anyone else. In fact no-one but film and tv critics ever use the term.
Film Critic: " X is a film depicting the gritty realism of life in the New York suburbs"
or
"the gritty realism of this documentary is in stark contrast to his other work"
So in a sense i'm right and i'm a sense i'm wrong.
Human Traffic is very light hearted BUT a great deal of the dialog is relatable to general reality.