I use off GCD skills on both BRD and DRG just fine. What I've noticed is you have to wait till half the GCD is done before they will activate. Watch your CD and try when it's half way. I think this is working as intended to prevent popping multiple skills off GCD while still keeping a perfect rotation. If you want more then one you will miss your next GCD. And sometimes it's worth doing that too. I think it's just fine how it is.