go to the Advanced Tutorial. there you get to try countering GB and also how to parry. with that you can find the timing. you'll learn to use it on reaction by simply playing the game though
Also if you play a class with a fast enough light attack, which most classes have, you can just use it when you see them close enough to guard break. If you are already doing the animation for a light attack at all it will take priority over a guard break.
I've always struggled with CGB, I tend to CGB when I think its coming so more of a prediction because on console I find that when I CGB it doesn't register quick enough so now i just predict it. Apparently on pc there is less latency issues so CGB has a bigger window.