Use PCGalerkinSetRestriction(pc,R) and/or PCGalerkinSetInterpolation(pc,P) followed by
PCGalerkinGetKSP(pc,&ksp); KSPSetOperators(ksp,A,....)
Developer Note: If KSPSetOperators() has not been called on the inner KSP then PCGALERKIN could use MatRARt() or MatPtAP() to compute the operators automatically. Should there be a prefix for the inner KSP. There is no KSPSetFromOptions_Galerkin() that calls KSPSetFromOptions() on the inner KSP