ok i think this is right but Greenwich is the closest "big town" to London or soemthing and its right on the longitude (the vertical lines you see on the globe) line that runs directly from the north pole to the south pole - and its considered to be line number 0 out of all the lines that go down around the globe - which is why countries to the east (the right) of Greenwich are GMT + and countries to the west (left) of Greenwich are GMT - thats more or less the basic explanation lol but its a bit more details than that
It is, & it's been that way since 1675, & was founded by King Charles II to solve the problems with longitude, The Royal Greenwich Observatory is built exactly on 0 degrees latitude, In the RAF, I was stationed near the market town of Louth in Lincolnshire which is on the meridian line & in the town square is a monument marking that line.