Less or fewer
I regret having to take issue with @PreciseEdit, but I’m pretty sure that this piece of advice on Twtter today is misguided:
Wrong: We have LESS than 3 gallons remaining. Right: We have FEWER than 3 gallons remaining.
You can see the logic: We use less for a mass noun, fewer for a count noun, and we are enumerating gallons here.
But that distinction between mass noun and count noun can be a little fluid. I think that when we talk in this context, we are not talking about three gallons as three units, but three gallons as a quantity, that is, a mass. So we’d say that there is less than three gallons remaining in the tank, just as we would say in the kitchen that we have less than three pounds of flour for this recipe.
If @PreciseEdit had used carrots or pieces of your mother’s stemware instead of gallons, all would have been well.