From Discovery Wiki

AGS (or A.G.S.) means After Gallic Settlement. Similarly to what developed into A.S., a new calender was created for Gallia shortly after reaching New Paris. 1 A.G.S maps to 84 A.S., and 733 A.G.S.- the year that Gallia was revealed to the rest of Sirius- maps to 817 A.S.

See also