How controller.isGrounded used for GameObject?

Unity Engine
,
1

How to change the controller.isGrounded

private var controller : CharacterController;
controller = GetComponent (CharacterController);
if (controller.isGrounded){}
@script RequireComponent (CharacterController)
@script AddComponentMenu ("2D Platformer/Platformer Controller")

to GameObject ?

var Opponent : GameObject;
if (Opponent.isGrounded) {}

It has error:
‘isGrounded’ is not a member of ‘UnityEngine.GameObject’.

2

idGrounded is just a shortcut for CollisionFlags.Below. It doesn’t make any sense for other objects, because unlike the CharacterController, they’re not forced to be aligned with the world’s Y axis.