Hi Rolia,
I have the exact same problems with my macbook retina from mid 2012.
I to get these grayscreen glitches after I open my laptop from sleep.
These glitches started maybe 2 months after my purchase in FEB 2013.
After Apple in 2015 finally recognized problems with videocards from their macbook retinas from 2012 I contacted Apple.
The first time I visited the Apple store in Amsterdam they ran their hardwarecheck software to test my videocard. Midway the test the screen froze and they took my macbook in for further research. When I picked up my macbook they told me they didn't find any further hardware issues during the test and they filed it as a software problem. They told me to wait for the next big update El Capitan where it should be fixed.
I told them I wasn't to sure it was a software problem as I've had the same glitch already in Mountain Lion and tried several clean installs to solve the glitch.
Anyhow, I trusted their expertise and patiently waited for El Capitan.
After installing El Capitan my macbook was glitch free for about a month, but after it got back hard.
I went back to the apple store hoping they offer me a fix that would permanently remove the glitch issue. They again tested my laptop, but ofcourse couldn't find anything in the hardware (ugh). Apple was not able to give me any proper explanation on what could cause the glitch, or how they could solve it. Better said they took zero responsibility for fixing the issue. I was told again it was a software problem.
I frustratedly left the apple store and tried a AGAIN a clean install to see whether it would solve the glitches (NOT).
And HEY what a surprise, a clean software install did NOT solve any of my 'software' issues, nor did any of the patches they promised.
More annoyed then ever I again went back to the apple store Amsterdam last week - refusing to be send away without a final solution from Apple. I got to speak to a manager who got the head of the technical department to help me. The tech guy explained to me he would start a chat with the tech department in the US to which European tech departments go to if they don't know how to solve a technical issue.
Ok, so now the super techies from Apple US would look at this glitch issue they would finally fix it... right??
Nope.
One day later I got a call from the head tech guy from Amsterdam that his colleagues in the states could not find any issues while running their hardware test software. YES YES I KNOW THAT APPLE'S HARDWARE TEST SOFTWARE ISN'T ABLE TO FIND ANY ISSUES. I KNOW!!!!!
All i'm asking Apple is to make the glitch go away...
Long story short,
Apple has yet to take any responsibility for this glitch problem and refuses to help me with a solution.
This glitch issue has cost me over 20 hours+ not to speak the 180 hours I've worked to buy this macbook.
Now I'm stuck with a MBR I'm not able to resell, because who in the world is going to buy a secondhand MBR that has a grey glitch screen when opening??!?!?!