Okay. Load this up, start a new game.
Summon FlashZombieSpawner and watch how the two light each other up and turn on bright lights.
The zombieman (FlashZombie1) has DontLightSelf on.
The shotgunguy (FlashZombie2) however, doesn't have DontLightSelf with his light.
Every 8 seconds it will restart.
The point is, the BRIGHT keyword originally had GL lights be ignored, regardless of this setting. So whenever the shotgunguy's BRIGHT keyword kicked in, it would light him up just as the zombieman should behind the duo in the pic below. To see an easier example, you could summon FlashZombie1 behind the group like here (make sure to freeze cheat):
http://puu.sh/c61h3/140807d48f.jpg
Long story short, outsider GL lights are ignoring the BRIGHT keyword.