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