In this paper we characterize the relative Gorenstein weak global dimension of the generalized Gorenstein Y-flat modules recently studied by S. Estrada, A. Iacob, and M. A. Pérez. As application we prove that the weak global dimension that comes from the Gorenstein FPn-flat modules is finite over a Gorenstein n-coherent ring and coincide with the flat dimension of the right FPn-injective modules. This result extends the known for Gorenstein flat modules over Iwanaga-Gorenstein and Ding-Chen rings.