You can cut about 6% off your GCD if you stack it on every piece you have.
The 30% likely applies multiplicatively. To get 100% uptime, you'd need to fit 9 GCDs in 6 seconds, which is a GCD time of 0.667 seconds. Assuming 100% uptime on Fey Light, that's 0.952 per GCD base, which is... let's see... something to the tune of 1889 skill speed. So yeah, that ain't gonna happen.