From Surnames of Ireland by MacLysaght

Gilbert-This name has long been prominent in Leinster, Gilbertstown is now found as a place-name in five counties--two (Louth and Westmeath) in the sixteenth century.

MacLysaght usually says if a name is Scottish, English, or Norman if he knows. In this case he does not render an opinion. I think we can safely say not Gaelic but perhaps in time more Irish than the Irish. Certainly prominent or at least prolific to have five towns of the name.