I contacted Gigabyte through a support ticket and they replied today:
"Dear customer,
Please temporary disable one core in bios to work with Win10 our engineering dept is looking into this issue, once new bios available will post on gigabyte website for update. Please check back on our website later.
Best regards,
Gigabyte technical support team."
So hopefully that pretty much confirms that they're working on the issue. However, I have found a fix.
1) Disable one core in the BIOS.
2) Install Windows 10
3) Boot up with one core and go to C:/Windows/System32/ and delete the file "mcupdate_genuineintel.dll" - you will need to use a registry tweak (
http://www.howtogeek.com/howto/windows-vista/add-take-ownership-to-explorer-right-click-menu-in-vista/) that allows you to take ownership of this file.
4) Once you've deleted this file, reboot and enable your second core in the BIOS. Windows 10 should now boot and work perfectly.
I've running a Gigabyte GA-H81M-H board with my G3258 now overclocked at 4.2GHz and it's working. Hope this helps.