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