Originally, if Gilbert and Ackerman were rescued they would wait at the entrance to the Boulder City ruins, unarmed, for the rest of the game. This was changed in a patch, causing them to simply disappear from the game world after a period of time once the Boulder City Showdown quest has been completed.
ps3 If Private Ackerman is killed during the quest and his limbs were blown off, you can find him back where he was alive, but he will not have the limb that was shot off. There are no Karma penalties or actions against the NCR if you kill him while he's in this state. [verified]