Well, cola is mostly water anyway - it may be a bit denser because of the sugar and other ingredients, but they'd probably weigh pretty much the same...
1 gallon makes 128 ounces and 1 ounce is 0.0078 gallon in U.S measure. There are 8 ounces in a cup, 2 cups in a pint, 2 pints in a quart and 4 quarts in a gallon. That makes 128 ounces in a gallon...