That is a locking device meant to secure your GPU in the slot when inserted properly. Push the end to the down position. When you insert the GPU (straight down), the lock will move upward and lock onto the tab at the back of your GPU connector. Good luck!
What you mean by "down position "? You mean push it towards the motherboard? But the thing is that the plastic is not moving a cm.... so I can't insert my gpu with the plastic is on the way. My gpu has this L shape and the plastic lock accepts
only rectanglar shape unless I move the lock out of the way somehow.