If the Vault Dweller chooses to kill Razor and/or make the Blades hostile, the Gun Runners become hostile, preventing access to this quest. So it's best to wait until completion of Exterminate the deathclaws.
Bug
After completing the quest, move out of the sector and run back to the Gun Runner's territory in less than 2 days. Then you may be able to gain 1000 exp each time you do so.