It seems that you are attempting a continuous convolution, not a discrete one. For lack of an NConvolve, you may try something like this:
cnvlv[x_?NumericQ, y_?NumericQ] :=
NIntegrate[f[\[Xi], \[Eta]] DiffG[x - \[Xi], y - \[Eta]],
{\[Xi], -Infinity, Infinity},
{\[Eta], -Infinity, Infinity}];
cnvlv[5, 1]
