Before you can get the Golden Gun, you have to find an audio recording. Once you reach the "Arboretum" area of the ship, locate the deep storage grav shift. Look for a sign with an arrow pointing left to an area called "Looking Glass". Proceed left to find the "Looking Glass Scene Selector" screen. Smash the screen, then climb up the small shaft using your Gloo Cannon. Next to Alex's house is a body with the Golden Gun audio recording that gives you access to Jada Marks' safe in the Crew Quarters. You can then follow the side quest marker directly to her safe and collect the Golden Gun.