This is most likely Micro BTX, which has been killed off. The upside is that Gateway often uses OEM Intel reference boards, which means that you might be able to find an OEM Intel board that is essentially identical to one of the boards used by Gateway.
Why is that an upside? If you can flash the Intel board with the latest Gateway BIOS, it would preserve the BIOS based OEM pre-activation.
Geeks has several cheap Intel Micro BTX boards, scroll to the bottom:
http://www.geeks.com/products.asp?cat=MBB
Or you can check
Ascendtech for used/refurb/surplus Gateway boards. Much of Ascendtech's inventory is used/refurbished even though it neglects to disclose it.
Oops, just noticed this may be an AMD board. Ascendtech might have some Gateway AMD boards.