No, F being a necessary condition of G simply means that if you have F, G will occur, which is not what you are saying. What you're saying is that G is absent when F is. You are not saying that G is present whenever F is (G can be absent whenever F is but it could also absent on certain occasions when F is present, or during all occasions when F is present). Furthermore, you are not certain that F is the cause for G. Just because two events are temporally contiguous, this does not mean that they are causally related. Unless you establish causality, even if you did establish the fact that G occurs whenever F does, this still does not mean that F necessitates G. There could be other variables that affect both F and G that, when combined in the right manner, produce F without G or vice versa.