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