According to myth - there was a dragon, by the name of Gargouille, who drowned the countryside around Paris in the 7th century. Appearently Gargouille rose from the waters of the Seine in France and, suiting to its entrance, breathed water and not fire! (which even at this time was considered the international standard for dragons)
Gargouille was, eventually, tamed by the Archbishop of Rouen, who led it back to the center Paris, where Gargouille was slain and afterwards burned. The so-called dragonslayers saved the head, as is custom in such an event, and mounted it on a building for all to see.
With this myth in mind, architects started designing gutter spouts resembling Gargouille, "breathing" water from their stone-cut mouthes when it rained. The sound of water passing through the insides of such a stone gargoyle and into thin air, is supposed to sound like an inhuman gurgle/gargle noise - This sound was later known as "the language of the gargoyles".
Some say the stone monsters still use this way to talk between themselves on a rainy night.