The equivalent of GameObject in Unreal is actually a component. Scene Component, to be more precise. However, components can only exist within an actor, with root (Scene) component within the actor providing actor's transform. The closest equivalent of "Actor" in unity is prefab instance root. Basically, imagine if "GameObject.transform" was optional, and BoxCollider component had a transform of its own. That's Unreal object model. Things get more fun when you start diving into ownership and packages.