Greenwich Mean Time (GMT) serves as what?

Greenwich Mean Time (GMT) serves as the basis of all world time because it was established as the mean solar time at the Prime Meridian, which is located at Greenwich, England. This makes GMT a reference point for timekeeping across the globe. Each time zone is defined in relation to GMT, which allows for a standardized method of measuring time in different regions. When one says a particular locale is GMT+2, it indicates that this locale is two hours ahead of the standard time defined by GMT. This system is crucial for coordinating activities across different time zones, especially in international travel and communication.

The other options do not accurately represent what GMT is. For example, while it is important in the context of navigation and geographical positioning, it is not related to measuring gravitational pull, distances on maps, or functioning as a geographical boundary. These aspects pertain to entirely different fields of study, reinforcing the significance of GMT specifically as a time standard.