In-universe, of course, and especially in the official Rockstar heists. In The Pacific Standard Job, Lester calls the guys "creepy, quiet weirdos," yet we constantly hear them communicating, even while Lester is giving them info about their job, often to the point where they don't know what Lester just said. Clearly, they're talking to each other in a way that people outside the gang can't understand: telepathy.
WMG / Achievement Hunter Grand Theft Auto Series
The Hunters can communicate telepathically.