Atri, most of the time when you hear about water to make a gallon, the discussion is about juice, or concentrate. Whole fruit, fresh or frozen, should be and most likely be refered to in a different scale.
If you are using frozen concentrated juice for example. Those are typically 12 oz. If you need 3 to make a gallon. You are starting with 36 oz. of concentrate and you will add 92 oz. of water to make a gallon.
As was just mentioned fruit and sugar wil displace some of that. I don't figure that into my original scale mentioned above.
Sugar will indeed take up some room, and should be considered. For example, if you are making 1 gallon of wine(any flavor, depending on the fruit, juice, etc.), you are looking at adding around 2- pounds of sugar. Although it is being dissolved it still occupies the space.
I think alot of new winemakers forget this when we are talking about a 1 gallon batch. Just because the recipie is for a one gallon batch, doesn't NECESSARILY mean it will fit into a one gallon jug. Some recipes take this into consideration, but most don't.
So in conclusion..if it says, "to make a gallon" it means your juice, etc, and enough water to make 128oz. Then use some of that juice(must) warm it up and dissolve your sugars in that. But guaranteed, it will increase the overall volume. be prepared for that.
Troy