The valve on the right-hand side of the picture is controlling where the water is coming from as it exits the pool and is pumped to the filter. Right now, it is drawing from 2 sources in the pool at about the same rate. It looks like one side might be throttled back just a little. You can try moving the valve all the way to block one pipe and see if you can tell what that stops. Then turn it the other way and see if you can tell what that stops. If one of the pipes is from the main drain, it will be difficult to know what happened. If one is from the skimmer, that should be easily apparent.
All that said, the behavior you describe sounds like either the skimmer weir is stuck, or you don't have enough water in the pool. Make sure the skimmer weir (flap near the opening of the skimmer) is freely moving. If it is operating properly and water is free to flow over it, add more water to get about 2/3 up the skimmer opening.
How long did you let the gurgling happen before turning off the pump? Did you watch it go through the cycle of filling/draining a few times?