I once seen a GB for 'Funk' button I am sure that fitted in somewhere.
As Rhyso said, it is German, and "Funk" translated means "radio". The switch is used by German emergency services for operating their two-way radios.

Not seen them in the Golf5, but are prolly used in the latest Passat, which is now based on the Golf5 platform. Audis also have a funk button.