Old topic, I know.
I've been wondering too the memory usage of my game and the only solution I've found is modifying MG and exposing the _resources field, then calculating the memory usage of each element. Not a trivial work.
However I've found an easy way to get a rough estimate of GPU memory used in Windows' Task Manager if you don't need that data to be accessed from the game: Just go to the Task Manager, Details tab, Dedicated GPU memory. If the dedicated GPU memory does not appear (usually won't appear) right click on the column list and select it.