I'm currently e-mailing with the technical staff of Gigabyte.
I also have exactly the similar problem with unlocking the X2 555 on a 880GMA-UD2H
I've tested the same CPU on a MSI 785GM-E51 board and it unlocks without any troubles and problems (and running really stable exactly).
On the latest reply to me, the Gigabyte staff was talking about a F5 version of the BIOS.
I can't find it, but maybe they were confused with the 880GM (without the "A")??
To be really honest, I'm a bit disappointed.
I had the choice to buy this board a a little cheaper board.
I chose the more expensive 880GMA-UD2H board because of the simple and better unlock function (according to the Gigabyte website).
Unfortunately, it isn't working at all