It is not one long single road. Your map is generally ok, but you will need to hit short sections of tar, to get to the next piece of gravel.
ChrisL can give you pointers regarding where to ride between Mosselbay and Stilbaai.
You will go over the pont at Malgas, which is quite scenic.
Once you hit Bredasdorp the gravel options becomes less. I would suggest you hit the 15 km tar to Napier and from there
take the gravel road to Elim and Baardskeerdersbos. From there, you will have 2 different options to get
over the smallish mountain and then you will pop out on the tar road a few kilometers outside Stanford.
From Stanford to Hermanus is about 25 km of tar. No gravel option.