How were these GPS points obtained?
The overview came from Transnet (https://www.transnetnationalportsauthority.net/OurBusinessUnit/LighthouseServices/Pages/default.aspx) where all of South Africa's lighthouses are listed. The GPS coordinates were then obtained by searching Open Street Maps. Further details were obtained from Wikipedia (on the year of construction).

Are there any known data gaps?
The height and year built must be properly verified. It would be good to get a description for each of them as well.