The spell is pretty straightforward even from the name, but let's see the description of the spell from GURPS.
Turns water or other drinkable liquid into a mildly alcoholic beverage.Yup, there is no confusion what this does. To Create this ability we first start with the Create advantage from GURPS Powers on page 92. But we aren't creating alcohol out of nothing. So we have to modify Create a bit. Since we are not creating alcohol out of nothing, we can take the transmute only modifier, and we are doing one transmutation type, "drinkable liquids to drinkable alcoholic liquids". This is from one category to a smaller category. Drinkable water is a specific item, according to the rubric for starting point values for the Create advantage, so I estimate that "drinkable liquids" is a small category which includes water. Since we are transmuting from something in a category to a smaller subset of that same category, the transmutation seems to be worth 50% value. Finally, a transmutation already costs 1 FP, so we use the magical modifier instead of the sorcery modifier. The spell shakes out as follows:
Water To Wine (-60%): Create (Alcoholic Drink) (Transmutation Only -100%; Drinkable Liquid To Alcoholic Drink +50%; Magical -10%) .
When cast on a little more than 1 gallon (or, exactly 10 pounds) of water or any other drinkable liquid, it is transformed into the same quantity of alcoholic beverage for 1 FP. Higher levels, if desired, can lead to exponentially increasing quantities of alcohol creation for the same price at 4 points per level. Depending on a different GM's take, if all drinkable liquid is a Medium Category for the Create advantage, they can double the cost of Water To Wine to 8 points per level.