GameObjectとgameobjectの違い

 
GameObject  is a type of object and acts as a base class for most Unity entities. It refers to the GameObject stored by unity which holds all Component data related to a GameObject (such as the Transform), the name and various other things.
gameObject  is a local variable of type GameObject which is inherited from Component. It allows one to access the instance of the GameObject to which this component is attached.

gameObject 

is in most cases functionally the same as

GetComponent("GameObject") as GameObject