Environmental damage is generally calculated by giving a value to a resource based on its availability. The more a resource with limited availability is used in an economy-- the higher the cost of environmental damage. So the calculation is basically geared toward availability and sustainability of natural resources.
I think that green GDP is far superior and realistic than regular GDP because it gives us a truer picture of what we are facing in terms of resources. It also encourages us to look for alternative resources that are actually sustainable. Green GDP encourages us to take responsibility for our actions and think about our future.