Welcome. GC stands for German Castrol which is Castrol Syntec 0w-30 (and only 0w-30 - all others are Group III and made in the U.S.). Unlike the other Syntecs, the 0w-30 is Group IV, or PAO stock, which many consider to be superior to Group III, which comes from petroleum. All GC has "made in Germany" printed on the back; regular Syntec does not. You'll also see much discussion about green vs. gold GC. Starting last year, Castrol changed the formula a bit and the newer stuff is gold. Look at the bottom of the bottle; all serial numbers starting with 0502 or higher are gold. Nobody really knows if one is better than the other, but people enjoy hunting for the older green stuff.
Finally, it appears that GC is best suited to VWs, Audis and Mercedes, however it has been used in many, many different types of engines with various results. Do a search about GC and Hondas and you'll see what I mean. Meanwhile, I've used Mobil 1 5w-20 and Pennzoil Platinum 5w-20 in my '03 Accord with great results. Do a search for 5w-20 UOAs; they are almost always great.