Then again, their nForce 570 board is advertised to support Phenom:
http://www.giga-byte.com/Products/Mothe ... uctID=2513
And at least MSI seems to think that nForce 550 could be Phenom compatible, since it is under testing in their nForce 550 board (and I think they are only testing if the power draw is suitable for the board, since the 125W processors won't work with the power design of that board):
http://global.msi.com.tw/index.php?func ... 3_no=#menu
AFAIK there is very little difference between nForce 550 and 570 besides the SLI, ethernet, RAID5 and number of SATA ports and none of those certainly is a requirement for Phenom to work. In other words they're just lying to their customers. Sad story. Looks like they're just wanting to sell more boards to people upgrading to Phenom.
I wonder what NVidia says about their chipsets' Phenom compatibility. If you ask them, don't mention what Gigabyte said, because if you do then they'll just stick to what their partner says or give some evasive answer.