2011 Macbook Pro - GPU dying?
Since people are still commenting on this, I will post what happened as my answer:
I took the MBP to a different store where they concluded that it was a faulty HDD cable since I had installed the SSD myself and they assumed it got some kind of shock for me not wearing an ESD bracelet. (Although as far as I can remember, the friend of mine who installed the SSD for me did indeed wear the bracelet, however I probably didn't use it when I installed RAM 1,5 years ago) 

It seemed to work fine for a while but after that, the glitches started to reappear, and it became so bad that the MBP couldn't boot into OS X or Windows (stuck on grey screen with glitchy stripes).

Once again, I took it to the shop, they concluded that it was a faulty logic board, which would cost me a fortune to replace. However, thanks to EU consumer laws, I have the right of up to 3 years warranty if I could prove that it's the manufacturer's fault. So they let me fill in a form and send it in along with my receipt. By the beginning of July, Apple finally approved to replace the logic board for free. For now, the MBP is working properly again.