You’re welcome. Thanks for bringing it up. I’ve wondered myself, and believe I’ve seen GTIN before. For even more explanation, there’s this:
where they seem to say that GS1 administers GTIN and takes into account all UPC, EAN, etc., and that 14 digit is needed in the future because they’re going to run out of numbers in the 12 and 13 digit UPC/EAN. I guess CD will finally have to go away at that point if there’s no way to encode 14 digits. 
Also, I’ve seen elsewhere talk of possibly dropping the trailing check digit from GTIN to go to UPC/EAN, but I’m pretty sure that’s not correct.
On all of this, I’m certainly not an expert. Just finding out myself, and open to correction.