It is a re-broadcast of the entire first season of Gilmore Girls. The WB tried this before with 7th Heaven and was successful at bringing new viewers to the show.
And, since WB owns "Gilmore Girls" lock, stock, and barrell, it's cheap. No need actually spending money on the shows you air opposite "60 Minute" and football.
I seem to recall they were planning to program rebroadcasts of other WB-owned shows ("Charmed" and "Gilmore Girls" specifically; maybe "Smallville") for Sunday afternoon/early evening, but I'm not sure whatever became of that plan.
They are still planning to re-broadcast the previous week's episodes of Smallville and the new show, Everwood (?), in the 5-7 p.m. time slot on Sundays. Gilmore Girls Beginnings will be from 7-8 p.m.