I thought everyone understood that GMT = Greenwich Mean Time.
http://wwp.greenwichmeantime.co.uk/All world times are + or - GMT.
Standard time zones can be defined by geometrically subdividing the Earth's spheroid into 24 lunes (wedge-shaped sections), bordered by meridians each 15° of longitude apart. The local time in neighboring zones would differ by one hour. However, political boundaries, geographical practicalities, and convenience of inhabitants can result in irregularly-shaped zones. Moreover, in a few regions, half-hour or quarter-hour differences are in effect.
Countries, such as USA and Oz will have there own time zoning because of their size.

If you want more info read the following:
http://www.answers.com/topic/time-zone