The Title of "GrandMaster" is one that means "The highest" or " The Top" in his or her field of profession. As a Chef, you are there to cook excellent food and and provide Elite taste and flavor using only the TOP quality ingredients.
To Become the GrandMaster Chef, you must know it all, from lvl1 recipes to lvl 70+ recipes. Wether you are horde or Alliance, having all 113 Cooking recipes, some acquired from horde NPCs (alliance mostly), and have don't beyond what any other chef has done and perfected your cuisine.
And while this isn't an in game Title yet, We hope blizzard will soon offer this wonderful trophy to the list.
See also: Grand Master Chef