Retina Mac shows gray screen when waking up
This is a workaround to save my work, then reboot.

Assumption: remote ssh is enabled and accessible on the Retina MacBook

Steps (note: "it" is the Retina MacBook):


Connect it to power, switch it on, and make sure it is connected to your LAN
Login over ssh to it through a terminal from another machine (I used a MacBook Air, but could be any machine that has an ssh client)
Enable screen sharing as described at Enabling OS X Screen Sharing from the Command Line
Remote connect through VNC to it 
Save all your work
Shut it down
Start it
Login
Install DeepSleep (or a free DeepSleep)
Force a Deep Sleep
Wake


It woke fine, so I hope this was a one-time issue.