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