You're right; the verb should be in the singular form: "Three gallons of ice cream is not enough for our party." The noun phrase "three gallons of ice cream" is a volume measure or quantity, so it should be treated as a collective singular noun. Even if the ice cream is in several containers, that noun phrase should still be treated as singular for as long as we are talking of the ice cream contained in them. Of course, when we are making a count of the containers of the ice cream, the verb should take the plural form: "Three gallon containers of ice cream are not enough for our party."
