Did you try to configure it? Are the commands available?
Personally I couldn't find any reference to it, or data sheets, and especially not in 12.0.
You can see here it was supported from 12.2, but no mention of the 12000 series.
http://www.cisco.com/en/US/docs/ios/12_2t/12_2t15/feature/guide/ft_glbp.html
Regards,
Ian