When puzzled, or confused, it is used in place of such phrases as "Huh?" or "What?" Unlike grrr, "Grrr?" is not said in an agressive tone, rather a questioning, perplexed tone.
Person 1: (Mumbles something incoherent)
Person 2: Grrr?
Look of adrenaline-fuelled determination on a person's face as they engage in some kind of extreme sport. Commonly used by whitewater kayakers. The Grrr face is said to resemble one's orgasmface.
"Haha... you had the funniest grrrface going over that drop - like the river was trying to hit on your sister"