Gameobject as Optional Argument

I was doing a major rewrite of some procedural generation code and I encountered somewhat of a brick wall. I need to have a GameObject as an optional parameter:

 public GameObject Generate( float radius, //These parameters generate a circular frag
                                int vertices, 
                                float degVar = 0.0f,
                                Color? color = null, 
                                Vector3? pos = null, 
                                Quaternion? rot = null,
                                GameObject? obj = null) 
 { 
 
         GameObjectfrag = obj.HasValue ? obj : (GameObject)GameObject.Instantiate (dupliFrag); //Instantiate prefab object if no object to be modified is provided.
 
 }

This is just the snippet of code that's affected. This throws the following error:

 Assets/FragmentGenerator.cs(29,46): error CS0453: The type `UnityEngine.GameObject' must be a non-nullable value type in order to use it as type parameter `T' in the generic type or method `System.Nullable<T>'.

I can't for the life of me figure out a clean workaround for this. One solution would be to use polymorphism, but I feel like there must be a more concise way to do this.