With Latest MacBook Pro and Radeon 460 Graphics, How To Tell How Much Memory in Use?
I would recommend using iStat Menus.

Here's a direct quote from their website:


  CPU &amp; GPU
  
  Realtime CPU graphs and a list of the top 5 CPU resource hogs. CPU
  usage can be tracked by individual cores or with all cores combined,
  to save menubar space.
  
  Plus, GPU memory and processor usage on supported Macs, and the active
  GPU can be shown in the menubar.


In addition to the above, iStat Menus monitors a whole range of things that you may find useful. 

You can download it for a 14 day trial to test it out prior to purchase.

NOTE: I have no affiliation whatsoever with iStat Menus.