It is now apparently impossible to download drivers from the Gigabyte download site.
No matter which download option is chosen what you get is the setup.exe for some Driver Update application for which one has to pay.
Eventually found the Intel Chipset Drivers for Windows 8.1 on the New Zealand Gigabyte site.
The headset now works correctly, but it does seem that USB on 8.1 is somewhat sensitive and unpredictable.
If I move the headset to another USB port then I have to reboot to get it to work.