Here are the current specs for GC Gold:
SAE 0W-30: Castrol SYNTEC 0W-30 European Formula is engineered to meet the Mercedes Benz
229.5 specification. The 0W-30 viscosity grade is ideal for winter conditions where low
temperature pumpability is required. A unique, low-temperature formulation provides exceptional
pumpability in cold weather and allows for unaided engine starts down to -40°F. Exceeds all car and
light truck manufacturer's warranty requirements for the protection of gasoline, diesel and
turbocharged engines where API SL, SJ, SH, CF or CD is recommended. Exceeds European
ACEA: A3, B3, B4; VW 502 00, 505 00, 503 01; MB 229.1, 229.3, 229.5; BMW LL-01; GM-LLA-
025, GM-LL-B-025 and the engine protection requirements of ILSAC GF-3 for API Certified
Gasoline Engine Oils.
Castrol USA Specs
As far as I understand it, ACEA certification is on the honor system via self-certification. That means the oilmaker produces test data that shows the oil meets the ACEA test requirements, but the data is not verified by in-house ACEA or independent testing. That's pretty much the same as with the API, I believe. This helps to explain why many carmakers continue to have and to develop their own oil specs.