A Green Documentation License would presumably put some barriers in place to turning contributed content into greewash, or perhaps using documentation for non-Green purposes that violate the six principles which all so-called Greens believe in, at least according to the Global Green Charter. There may also just be a simple set of restrictions that prevent its use for war and ecological devastation. At least, to the degree that any license can do that.
A GDL is clearly not Free according to Debian. But of course, who cares?