See if this will work... It is mostly used to unlock the x2 550 'disabled core' with your motherboard model, but it might help you find that missing core of yours
Hope it helps!
1. DISABLE "AMD COOL & QUIET" setting under Advanced CMOS setup. This is always enabled by default in every BIOS of all Gigabyte motherboards I owned
2. Set EC Firmware Selection to "Hybrid", Advanced Clock Calibration to "Auto". This has an equivalent feature in ASUS motherboards, and this is the key to the extra cores.