Unlocking the Desktops, Special Music, and Tonko Art Gallery
     To  unlock  these  features,  you  must pass all 7 Metal Slugs on
limited  continues,  varying  through  all  the difficulty levels. The
character that you pass each game with does not matter.

Unlockable                        How to Unlock
Tonko Art Gallery                 Pass all 7 Metal Slugs on 
                                  Easy Difficulty / Limited Continues.
Desktops 1                        Pass all 7 Metal Slugs on 
                                  Normal Difficulty / Limited Continues.
Desktops 2 and Special Music      Pass all 7 Metal Slugs on 
                                  Hard Difficulty / Limited Continues.