Wrong: We have LESS than 3 gallons remaining. Right: We have FEWER than 3 gallons remaining.
We bought a dozen cupcakes for the party. I just ate one, so now there are fewer cupcakes.
We bought a big cake for the party. I just ate a slice, so now there is less cake.
I have bought some milk. I only had enough money to buy two bottles, so we have fewer than three gallons.Why anyone would feel the need to point that out, I don't know. But if you were referring to the presence or absence of a number of gallon-sized units of a particular liquid, there is a case where you could conceivably use the word "fewer" in referring to them.
"How many gallon-sized bottles of milk am I holding up?""Four?""No, fewer."