The combat logs show separate events for "cast started" and "cast finished", with instant casts only having a "cast finished" event. So it's not too difficult to just starting counting seconds from any "cast finished" event and see when the next cast begins. Compare that to the GCD to see how much time you wasted in between.
But, I forgot about haste reducing your GCD, and since there's no way to see your haste rating from these combat logs I think I'd just have to guess what your GCD is supposed to be? (Not to mention haste being buffed partway through the fight, etc.)
My understanding is that with Legion, the combat logs will get a bunch of information added about your stats, talents, glyphs, etc., so I think I'll delay this feature until then so that I can ensure the numbers are accurate.