There is serious question about whether the G-Spot exists and what it really amounts to. In Golden Posts you will find an old discussion of this.

Dealing with the G-Spot is a rather sophisticated sexual technique. Forget it for now and perhaps after you each know what is going on you can start the most interesting exploration that goes with it.

You can see some diagrams here to see what some different types look like. Some you can put your fingers through, and some you cannot. If the sides of your fingers rest on it, then it may cause discomfort for her, or possible bleeding trying to insert them. If she has masturbated before, then it could be likely that she does not have one anymore, as many girls these days do not have a full hymen, but rather, only a small ring around the vaginal opening (still enough to cause discomfort during her first time having sex, definetly, though, but not as bad as if it is almost covering the whole opening).