Late 2012 iMac massive screen issue

[Heresiarch]

After some gaming my iMac's left third part showed this kind of mass vertical lines. They are practically all over there and are most obvious under grey screen. In the screenshots below, the cursor is exactly at the splitting point.

I don't think this is a image retention issue because I have no such screen with this kind of lines. I suspect it's a faulty LCD or GPU or worse. I'm scared at the thought of bringing in my iMac to the Apple Store to repair, not to mention all the files that need to be moved.

Edit: the thumbnail apparent got the direction wrong

 

[Heresiarch]

Small update: after an hour of no heavy GPU usage (i.e. games) it seems the lines have disappeared somewhat. They ARE still there but not so obvious. I hope this is only an image retention issue but rationally I know it is not that simple and the situation WILL become back and worse. And possibly soon too.

Good thing it happened now, 28 days before my one year warranty being off. So I immediately bought the Applecare. I can't imagine how much headache and waste of extra money I'll have when the machine eventually kicks the bucket if I don't have AC.