Now let's figure out how longitude works in galactic coordinates. Think of the Earth again. How do you measure longitude on the Earth?
Lines of longitude on the Earth are oriented north and south. They all pass through the North and South Poles. 0o longitude runs through Greenwich, England. 0o is the Prime Meridian. If you travel eastward from England 60o of longitude, you arive at Orsk, Russia. Orsk is at 60o east. If you travel westward 60o from England, you arrive in Goose Bay, Newfoundland (Canada). Goose Bay is at 60o west.
In galactic coordinates, the Prime Meridian, or 0o is at the galactic center.
Look at the Degrees Longitude diagram. See the bright galactic center and the 0o line that runs through it. Instead of going from 0o to 180o east and 180o west, galactic coordinates simply go from 0o to 360o. There is no east/west or plus/minus in galactic longitude coordinates. Remember that 360o is the same as 2 x 180o, or 180o is half of 360o. Notice on the diagram that directly opposite of the 0o line is the 180o line.