The default ebuild von gnome-mount does not build the nautilus extension. The attached patch fixes that.
Created attachment 82870 [details, diff] gnome-mount-0.4-nautilus.patch add useflag for nautilus and raises to -r2
This seems to be a bit problematic - it would make gnome-mount depend on nautilus which itself depends on gnome-vfs which depends on gnome-mount. For that reason they want to make it an extra package ( IMHO this is so basic that it would belong into nautilus itself ).
Don't think this can go in as it is, with the circular dep and all. A possible solution to just create a fake ebuild gnome-mount-nautilus which just installs the extension. Since the extension itself still seems a little rough, it's not high prio.
Will look into this for later releases when it is more useful and developed.