Successfully complete Story mode with Marco to unlock the Monolith with infinite ammunition and the Extras menu (with "Mini Game", "Color Edit", "BGM Test", and "Movies" options).
Collect the CDs hidden in the various locations in all missions. Then, enter the "Garage" screen and select "Laboratory". The staff will be found inside.
Add the Gunner Core, Gunner Arm, Gunner Cannon, and Gunner Leg to Metal Slug in the Slug Laboratory.