Here is the full list of all 29 Gunmetal Arcadia achievements.
Left the Seed of Unmaking
Kept the Seed of Unmaking
Completed a mission as Starling
Killed Brant
Completed a mission as Thrush
Gave the Seed of Unmaking to the Gunmetal Vanguard
Resolved Wren's quest
Unlocked the Wayfarer
Completed a mission as Grackle
Completed a single playthrough
Gave the Seed of Unmaking to the Seekers of Arcadia
Killed Anathema
Resolved Vireo's quest
Joined the Gunmetal Vanguard
Joined the Seekers of Arcadia
Continued a legacy
Fell in combat
Killed Amaranth
Killed Rail
Destroyed the Seed of Unmaking
Resolved Starling's quest
Resolved Grackle's quest
Resolved Thrush's quest
Resolved Gannet's quest
Resolved Jay's quest
Found the iSigil
Found the Codex Cadentia
Completed a mission as Vireo
Completed a mission as the Wayfarer