You are right about Southern Dunes being a gated development.

Upon arrival you would drive to the gatehouse and normally present some form of booking confirmation, picture ID and the address of the house. The guard would normally have your details on a corresponding list.

It has been some years since we stayed on SD so I don't recall whether they then give you a card to display on your windscreen (certainly this is the process with many other gated areas of non-residents) or whether they just get to know your car and wave you through on future arrivals.

For guests, just tell the guard who is coming and when and they will just add their names on to their list.

My parents have a house here, its a security gatehouse, you let them know where you are staying and they give you a pass for your car window, I have never had to show any ID or confirmation, but its perfectly safe there and a fantastic base for your holiday.

Feel free to pm me if you have any questions.