It's handled by Play services which is whitelisted to allow network access when the device is in light/deep doze mode. Play services can also instruct Google's

It's handled by Play services which is whitelisted to allow network access when the device is in light/deep doze mode. Play services can also instruct Google's servers to stop sending notifications when the device is in deep doze and re-open the connection when the device is active again -- thus saving power (messages are queued at Google's servers)