GCDDisplay shows a bar with your current global cooldown.
It can be dragged around screen by using the right mousebutton on the displayed bar.
There are a few slash commands:
/gcd lock - toggles whether the bar can be moved or is fixed in place
/gcd show - toggles between always visible, or only visible when active
/gcd info - shows summary of current settings
/gcd cdreset - resets the current GCD monitor
/gcd bg - toggles display of the black background bar
/gcd cast - toggles visibility when casting
/gcd number - toggles display of numeric timer
/gcd w XXX - sets width to XXX (>=50 and < screen width)
/gcd h XXX - sets height to XXX (>=1 and < 100)
NOTE: The bar represents a timer that is your default unmodified GCD duration. So, if you have something that modifies this, the red bar will start somewhere along the black bars length - the important part is when the red bar is reduced to zero length.