Generally speaking...these are known as "production releases", ususlly anytime within 2 weeks of a show, and often the day of or day before a show some good seats will pop up on ticketmaster and at the box office. This happens for most concerts, not just U2. Often these are excellent seats as they have been held back for guests of the band, promotion etc., and the promoter realizes he doesn't need some, they get released back to the public.
As for GA seats for U2, it seems that they are popping up only the day of show on the 3rd leg...you probably would need to camp out at the venue box office.