arl = Reverse @ Thread[Gs -> (a _ # & /@ Rg[12])]

{x y x^2 -> a _ 12, x^2 y x -> a _ 11, y x^2 -> a _ 10, x^2 y -> a _ 9, y x y -> a _ 8, x y x -> a _ 7, x^2 -> a _ 6, x y -> a _ 5, y x -> a _ 4, y -> a _ 3, x -> a _ 2, 1 -> a _ 1}

Converted by Mathematica  (October 16, 2002)