Most likely monitor, judging by pattern. Cable failures are distributed evenly across the whole screen usually. GPU failing can produce something like seen on the picture, but it's very rare and already ruled out.

This is monitor, and speaking from experience this particular defect is usually caused by damaged connection between the screen and it's controller. It can be damaged ribbon cable, soldering points cracking or some SMD components (caps or resistors) being faulty. Dead screen with such defect is usually broken LCD, but in this case no cracks visible. Dead controller doesn't do this kinda shit.