Here is the full list of all 11 Gunmetal Arcadia Zero achievements.
Finished a speedrun
Finished a speedrun in 30 minutes or less
Joined the Seekers of Arcadia
Started a New Game+ file
Completed Chapter 6
Completed Chapter 5
Completed Chapter 4
Completed Chapter 3
Completed Chapter 2
Joined the Gunmetal Vanguard