If it was your GPU, you'd see corruption in the openfirmware prompt and the machine would probably crash the moment you went into a non-VESA screenmode. Another telling issue is (if your system supports it) that HDMI/DisplayPort sound would be missing or badly crackling.

I had a rather expensive Radeon R9 fail on me just that way. One day, computer just shutdown, and wouldn't boot into Windows or Linux, but would work on Windows safe mode. There was ASCII trash all over the bios.

Managed to get a cheap second hand replacement, but I almost had the same thing happen twice. Seems my optical drive was dirtying its power; giving it two molex straight from the PSU cured the problem and it hasn't come back. Probably some brownout or surge fried the VRAM.

But like I said, you'll probably know GPU or VRAM damage if that happens, as it makes ordinary use of the system impossible.

Such issues tend to happen quite a lot on newer Mac notebooks, at least if you believe Louis Rossman. Poorly designed track layout and cheap components sometimes allow bridging of high voltage into CPU and GPU lines, with predictable results. Steve Jobs was of the differing conviction that Apple hardware should never be shit, so the older PPC stuff is much better built.

But yeah, just sounds like one of the LCDs has gone in your display, that's all. All the pixels in series with it will be effected. There's no fix, but at least you only have to change the display.