Hi guys,
Here is a link to get directions from Johannesburg to Gariep - close to Colesberg if you are looking to stay-over at Gariep Dam.
https://docs.google.com/document/d/1HbrhQrQ0vdKywpsm7S5TqHfvgxGi6JCZudqTqGXXrwo/edit
Siloam Village www.siloamvillage.co.za has made the document available since it helps a lot if you maybe forgot the directions at home.
Enjoy