http://mirror.centos.org/centos/5/updates/i386/RPMS/ i can see the kmod-gnbd for 2.6.18-8.1.8 fine there
It's not one of both kmod's, it's the combination ;) I can't find a kernel that has both.
Also, i dont seem to recall us ever shipping kmod-gnbd for the CentOSPlus kernels.
Well, that explains. Can you start shipping kmod-gndb in the CentOSPlus kernels?