Besides Intel, G35 was used only by ASUS. Intel has had trouble producing enough of particular IGP chipsets from time to time, that could be a factor here.
FYI, the Gigabyte board implements a PCI Express x16 slot that is actually limited to PCI Express x4. Gigabyte uses 4x PCI Express lanes routed from the ICH10 Southbridge to create this slot, instead of using the native PEG x16 interface in the G45 Northbridge. This also makes the Gigabyte PEG slot PCI-E 1.x rather than PCI-E 2.0.
The reason Gigabyte would do this is because Intel IGP chipsets are seriously limited in PEG + IGP configurations. The native PEG x16 interface in the Northbridge is literally wired (multiplexed) into the IGP in order to support ADD2 SDVO add-in display cards. Because of this, the IGP cannot co-exist if more than x1 lanes of the PEG interface are utilized.
Gigabyte works around this by using four PCI-E lanes in the ICH for the PCI Express x16 slot, which allows far more devices to co-exist with the IGP and still has more bandwidth than the PEG interface restricted to x1. The downside is for persons who want to use the native PEG interface at full potential and don't care about the IGP.