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In order to obtain the Gus you must first defeat Krubis. All of this can be completed following these steps:

Head to the Bounty machine and select the Krubis Bounty then head through the portal to "Zephyr Paradise," the planet with Krubis' base.
Pay a visit to one of Krubis' associates (Ranchy) in the mansion who provides you with a device capable of creating teleportation warps. After finding Krubis' warp location you use the device to bring him to your current position.
Once Krubis arrives, engage in a battle to defeat him.
Following Krubis' defeat, acquire his DNA. During this process, you'll also have the opportunity to obtain Gus, the Talking Shotgun, from his remains.

This achievement is unmissable so no worries there!