If you look at the actual dimensions, the GC size is closer to a 00, and the GA size is slightly larger than a 000. It isn't a perfect conversion. I used to think that GC was a 000, but digging into the dimensions, found a 00 to be closer. I too like the Taylor system, but as a recovering owner of Brand M, I had to know about all this.
The Martin system starts with 0 (zero) for the smallest common guitar. (For the purists, there are size 1 and size 2 models too, still smaller than 0's). Every time you go up in size - a wider body - add another zero. Guitars got bigger to be louder and hopefully compete with ensembles in larger halls. 0 = parlor, 00 = GC, 000 = GA, 0000 = J. In Martin nomenclature the J (jumbo) is the same body size as a dreadnought, but with a pinched waist, so basically an extra wide 000. Other numbers like 15, 17, 28, 35, 42 etc refer to wood types, trim level and bling. A 000-28 has the same trim and woods as any -28 model regardless of size. It get further confusing in that Martin (and the industry) is not consistent with 000 versus OM. OM or Orchestra Model was the first "large" guitar sort of born in parallel, with the dreadnought. Both OM and 000 have the same body footprint. Originally, the OM had a longer 25.4" scale, while the 000 models were short scale at 24.9". But over the years, Martin has made both models in both scales, so it is not a hard-and-fast rule. And it is pronounced "triple-ohh" but it is really triple zero. Now back to my corner.....