MG question

Hi I hope that this question is not outside the scope of this mailinglist. As far as I understand PETSc uses preconditioned GMRES(or another KSP method) as pre- and postsmoother on all multigrid levels? I was just wondering why and where in the literature I can read about that method? I thought that a fast method would be to use MG (with Gauss-Seidel RB/zebra smothers) as a preconditioner for GMRES? I have looked at papers written by Oosterlee etc. Kind Regards