The gurgling is due to large bubbles of air entering the bottle. What happens is this: first some water flows out, reducing the air pressure inside the bottle till it is lower than that of the air outside. The water then flows with less force, and air enters through the neck, checking the flow for a moment. The balancing of the pressure inside the bottle allows more water to pour out and stop air entering till the air balance is again upset, when more air gains admission. And so on till the bottle is empty.