An interrupt and a spell causing a GCD are not the same thing.

An interrupt will cause another caster to lose their spell when you hit them.
Your interrupt could be instant cast, or it might have a cast time.
If you cast Flame Shock (an instant), you still cannot cast a spell interrupt or not, until you are off of the Global Cooldown, which is an entirely separate issue.

Some spells don't trigger the Global Cooldown.
So for example you can put two spells into one macro without a castsequence:
/cast Elemental Mastery
/stopcasting
/cast Lava Burst

Not triggering the GCD is not the same thing as it Interrupts an opponent.
As far as I know, all of the interrupts do in fact trigger a GCD.
We have Kick, Counterspell, Windshock, Silence etc.