GAZ coilovers have the same fitment at the top of the rears as the standard shocks.
Either the above is a generic picture, or they are not Sierra ones.
You Don't need a rear strut brace to fit them, however, because you are putting the rear car weight where the shock mounts rather than where the original Spring sat, it is recommended to fit a strut brace to help stiffen the mounting points and spread load a little.