Taken from 2011 Macbook Pro Graphics Card FIX 100% WORKING!!!
EDIT This method works! But there is an improved version for better thermal management and brightness keys functionality. See improved version -> https://gist.github.com/cdleon/d1eff7246a25193304284ecec40445b0
if you are on high sierra 10.13.6+ you might need to use
Command + r
instead
Boot up holding down Command + r + s