If you have recently updated arch Linux you may face a flickering issue when system boots into GDM (login window) however the flickering is gone after successful login , after few searches i have found the solution.
You can see more details about this bug here : https://bugs.archlinux.org/task/44779 (caused by Linux 4.0.1(x86_64) kernel)
To fix the issue
uncomment the line
then restart the system.