I just call it a "Per-(base recast time) GCD". Gnashing Fang is therefore a "Per-30 GCD".

If it cannot be used during the GCD and triggers the GCD, it's a GCD. Historically, all skills which cannot be used during the GCD have also triggered the GCD.
If it can be used during the GCD and triggers the GCD, as Empyreal Arrow did in HW, it's usually been called a "Half-GCD", "pseudo-GCD", or "woven-GCD"
Anything else is an oGCD.