This is an editor / development only command. When you package all of the debug commands, like this one, are stripped from the game. What should happen is all the Materials in the level should go grey, however I have not tested this on a complexe sceen as if I have to do this type of testing I use something like NVIDIA Performance HUD or Intels GPU analyizer. If that does not work please let me know and I can look into something else for you.