3785.41178g (rounded during steps). You're able to find this by first converting a gallon to a liter (US gal=3.78541178 liters), breaking that into mL (3785.41178). A milliliter of pure water (hydrogen oxide, no minerals or anything, which water almost always has) weighs exactly a gram. Therefore, 3785.31178mL of water weighs 3785.41178 grams.