Where does a screenshot come? I've a problem with my Mac Mini.
Sometimes, from some time, it displays some glitches, or "graphical problems", for instance the screenshot below shows some bands in a part of the screen with black or "rainbowed" flashing pixel and sometimes, when Mac wake up from sleep, all screen has a green tint. Everything solves sleeping and waking up the Mac.



I was convinced that it's a GPU problem but I went to Genius bar with these screenshot and they told me that "screenshot comes from the screen, not from the GPU, so, if glitch appears even in screenshot, problem is in the screen or the cable, not in the Mac", is it true?
Your Genius was misinformed. When graphical artifacts or distortions show up in a screenshot, they are a product of the graphics card. It is one of the quickest tests, we did it all the time to classify graphics issues. Most geniuses know this, like I said you got someone who was either new or off their game that day.

Long story short, you're absolutely right that it's the GPU.