1) You have to be in the mission No Hope in Hadley's - This it the fourth mission of the game
2) The current objective will be to Get to the Comm Tower to Find Bella and Reid.
4) You get to the Communications Tower Doors. These are locked at first, but will open once you have killed all aliens in the area:
5) Go through the hallway:
6) Turn left:
7) Gormans Pistol will be on a table to your right:
Pick it up before advancing to the next area