from Wikileaf:
G-13 is believed to have originated in a research facility in Mississippi in the 60s or 70s, where, with the help from the University of Mississippi, pure Afghani Indica clones were used to attempt the creation of a super Indica strain. As the story goes, Neville Schoenmaker convinced someone working there to bring him a cut from one of the strains, labeled G-1 to G-23. G-13 was chosen as the most stable. This strain flowers for six to eight weeks to produce a reportedly high yield and works very well in a Sea of Green. Unfortunately, true G-13 is a rare find as it is available in clone form only.