The phrase "empty gallon" refers to a gallon container or bottle that does not have any liquid inside. It means there is nothing in it. Full definition
We like to use the bottoms of empty gallon bottles, but any dish will do. (saveastrayfoundation.org)
And if I would put some of my wall paint in cute mason jars then when I need to touch up the wall, like I do now, I wouldn't have to hunt for an almost empty gallon can. (masonjarcraftslove.com)
Their heads hung in shame and embarrassment as they handed us empty gallon jugs for water. (rachelheldevans.com)