I have included a link to a thread that has some ideas on how to potty train your grey, you might have to overlook some off topic chatter but otherwise it has some good info for you.
Some greys are easier to teach than others, some don't want to go potty in their cage and others like mine don't really care where they go, if they have to poo they are gonna drop it wherever it lands.
I have had some success with training my Josey to go in a trashcan, she rarely ever poops on me anymore but most anything else is fair game.
But remember that even if you succeed in training your grey to poop on command or go a certain place they will still have accidents from time to time, its not a 100% thing.
http://www.greyforums.net/forums/training/109-potty-training.html#109