GC is great when you can benefit from an oil that is almost a 40w at operating temperature, or because it meets certain manufacturer specializations such a VW502 and VW505. Also, it is API SL and I think it doesn't meet ILSAC GF4.
Edge is great if you need a different viscosity than GC. Depending on what date the Castrol Edge was manufactured, it meets API SM and ILSAC GF-4 or it meets SPI SN and ILSAC GF-5.
I have Gold Edge 10w30 in my car, and I will be using it for a while because I have enough for 3 oil changes.