GCD is an addon that provides a cast bar, global cooldown bar, swing timer and cooldown timers in a circular fashion around the mouse cursor or alternatively at a fixed position on the screen.
If you find a bug or have any wishes for the addon feel free to write a ticket.
Keep in mind that a lot of addons mess with the default castbar.
Should the toggle for it not work properly its most likely that another addon modifies it as well.
To open the options menu type "/gcd"