Gurgling or more likely steam/bubbles in the overflow tank would be normal if you are running straight water.
When the engine is at normal operating temperature (say going down the road) for 15/20 plus minutes the engine will be at around 200/205 degrees. Water boils at 212 at sea level. At say 3000 feet it boils at about 205 degrees, just about where the engine is running. A good radiator cap will raise it back to about 210/215.
So, to get to the point, when you shut down the engine, coolant stops flowing and removing heat from the engine. So, the water temperature in the engine goes up (a lot). Well past the boiling point of the water, so it makes steam. The high pressure steam/air then escapes from the system through the radiator cap (think of the old movies with a car going up a mountain and the radiator steaming) and into the overflow tank. And that is the gurgling you are hearing. When the engine cools off it may also appear that you have lost coolant because the radiator is a little low (not much) and the overflow tank may have a more in it. That would be normal.
Once you determine the cooling system is working correctly (pump, radiator, radiator cap, hoses, etc.) and you put 50/50 percent antifreeze in it the gurgling should stop.
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