Hi,

this looks more like a great-black backed, it appears that great black-backed gull is present at Khnifiss on a regular basis also, next to the kelp gulls present. Several pics of the species are deeming up on the internet...
If you look at the paper of P. Bergier in Birding World in 2009, you will clearly notice that kelp is present at the place, but one should be careful...

http://www.go-south.org/papers1/bergier_&_al_2009_cape_gull_birding_world.pdf

grtz
Pieter