I can look into it. The problem is it will have to scan your spellbook while in combat because the API call that I am aware of only gets the current GCD.
Anyway, I could do the math instead but it appears to be off, I assume rounding errors, for example on my booking when I get the GCD via the API it states 1.354 when I d the math it is 1.338 (100%-10.76%*0.015), but there are also issues with hybrids. An outlier case is Druids. Cat form has a static 1 second attack speed and GCD. Not particularly an issue for those in the know, but there may be other cases with other hybrids wher it is an issue, but I do not know, I will have to look into it. I can put this on the Haste stat tooltip but it may cause more questions than provide answers, heh.
In the interim here is a macro you can use. Replace Moonfire with a spell that activates the GCD and it will show you the GCD. it may be different between spells and melee attacks, etc.
/script local _,gcd=GetSpellCooldown("Moonfire"); print("Current global cooldown is "..gcd.." seconds.")