It wasn’t too long after the Nexus 6 came out until some users on XDA found out that it was capable of having the double tap to wake gesture. This could be enabled through custom kernels or via apps with root privileges. Google apparently scrapped the idea of double tap to wake in favor of its Motorola-esque Ambient Display.
Well, fast-forward a year and it seems like Google has done the same with the Huawei Nexus 6P. XDA recognized developer flar2, who is the creator of the ElementalX kernel, found it within the code on the device.
echo 1 > /sys/devices/soc.0/f9924000.i2c/i2c-2/2-0070/input/input0/wake_gesture
Currently, the only way to enable the double tap to wake on the Nexus 6P is to install a kernel that supports the gesture and has the code enabled, like ElementalX. Now that flar2 has discovered this, it is only a matter of time before more kernels will have this feature enabled. The same goes for an application, like Nexus 6 Double Tap to Wake, it’s only a matter of time.
If you are hoping that Google will enable this in a future update, you’d be better off with a custom kernel. Google is in favor of using Nexus Imprint and or Ambient Display over double tap to wake. At least they still include it on their devices for future development.