FreeGadget() is still needed?

Why can't the GC just delete this window set to Null? We still have to use FreeGadget()?

win:tgadget=createwindow("",200,200,400,300)

win=Null

GCCollect()

Notify "Is the window still here?"

Notify "Yes it is."

Close the window.

I don't want to.

Hide the window.