Short answers, pulled from the story.
"Mean" refers to the arithmetic average. Noon GMT is the annual average moment at which the Sun crosses the Greenwich Meridian, because Earth's elliptical orbit and axial tilt cause the actual crossing to occur up to 16 minutes before or after noon on any given day. This discrepancy is described by the equation of time.
Greenwich Mean Time was legally adopted across the island of Great Britain in 1880. The Railway Clearing House had used it since 1847 and nearly all railway companies followed by 1848, but a court case in 1858 still held local mean time to be the official standard until the 1880 change.
GMT was superseded as the international civil time standard on the 1st of January 1972, when Coordinated Universal Time (UTC), maintained by an ensemble of atomic clocks around the world, took over that role.
The name Universal Time was introduced in 1928 to distinguish GMT counted from midnight from the older astronomical convention that counted GMT from noon. The two conventions shared the same label but differed by twelve hours, creating ambiguity in scientific and civil use.
Countries and territories that use GMT year-round include Burkina Faso, Ghana, Guinea, Guinea-Bissau, Iceland, Ivory Coast, Liberia, Mali, Mauritania, Senegal, Sierra Leone, Togo, The Gambia, and Saint Helena, Ascension and Tristan da Cunha, among others.
On the 1st of January 1925, astronomers adopted the civil convention of counting hours from midnight rather than noon, creating a discontinuity of exactly 12 hours. The instant previously called "December 31.5 GMT" in 1924 almanacs became "January 1.0 GMT" in 1925 almanacs. The term Greenwich Mean Astronomical Time (GMAT) was introduced to refer specifically to the older noon-based convention.