By the way, I suggest to increase the timeout to 10 sec. Just a note, has nothing to do with your problem.
As for GC, I found this: http://ubuntuforums.org/showthread.php?p=10340183
When /etc/default/grub has been edited have you saved the file then run sudo update-grub?
Please post the output ofCode:sudo cat /boot/grub/grub.cfg | grep "menuentry" | cut -d '"' -f 2
MacBook Pro 10,1 retina
Hello guys, sorry for posting so late, I just wanted to tell you, that I already solved this problem with my pal, neither one of those softwares (SatrtUp manager or Grub Customizer) worked for me so we simply ran Terminal and typed in "sudo gedit /boot/grub/grub.cfg" and adjusted it manually including the resolution. Thanks a lot for your help anyway!