Google Certified Devices is any device that has GMS (Google Mobile Services) installed - ergo almost all of them. It's worth noting that a _lot_ of apps stop functioning when GMS is missing because Google has been purposefully been putting as much functionality in them instead of putting them in AOSP. So you end up in a situation where, to make an Android phone compatible with most apps, you need GMS. Which in turn means you need your phone to be Google Certified, and hence must implement this specification.