Yes, you can compare them. But you can compare apples and pears as well.
GS: Bsp / ABT based, Mesh / Model mixture Unity: Model based without any Scene Management
GS: CAD like editors Unity: Realtime Editors
Two absolutely different basics. Both not bad. It depends on the project if you really want to compare them.
Further: You would have to do the same project with both engines, if you want to compare. But that would be no fair comparison, because it depends on the project. For some projects Unity is the way to go, for some projects you would choose GS, and for another one Unreal.