No, you cannot: GF phi means that phi holds infinitely often on a path, and thus, GF GF phi does not say more than that. However, X phi says that phi holds at the next point of time, and X X phi means that phi holds two points of time after the considered one. That is not absorbed, and has a different meaning. However, you have that G G p is equivalent to G p and also F F p is equivalent to F p.