In order to gain this, the ultimate weapon in the game you must meet these requirements:
-Never trade out the
Polar Star for the
Machine Gun-Have the Booster 2.0
Earning the Booster 2.0 -Major Spoilers-
*** Spoiler - click to reveal ***Keep Professor Booster alive by skipping him after he falls down the pit in Labyrinth B (Can be a difficult jump), when you return to Arthur's House after exiting the Main Artery, Booster will teleport in and give you the Booster v2.0.
Return to where you collected the Polar Star and talk to the Gunsmith who is now awake.