Here is the full list of all 54 The Gate achievements.
Beat King Belial in IC2
Beat Duke Vual Of Ice in Fridgem
Beat Nero Glint Flesh in Ventum
Complete all battles in He-Elzapop
Beat Gaap Great Glare in Fumphzen
Beat Astaroth Fire Tail in Deca
Beat Corroding Vepar in Taglamig
Beat King Purson in Talvi
Beat Marquis Naberius in Danker
Beat Raum the Caustic in Spidra
Beat Prince Orobas in Synistu
Beat Bune of Inferno in Letea
Beat Blinding Shabriri in Angster
Beat Prince Samael in Desacrex
Beat Irvene Ice Tears in Doomus
Beat Baron Nergal in Sinistra
Complete all battles in Articum
Complete all battles in Subtyranny
Complete all battles in Hoarhole
Get 10 Friends in Game
Win 10 PVP Arena Battles
Win 50 PVP Arena Battles
Win 100 Arena Battles
Win 200 Arena Battles
Use friend cards in 50 battles
Finish 5 pages of Collection
Finish 10 pages of Collection
Get 5 Win Chain in Arena
Get 10 Win Chain in Arena
Train 20 Skills
Win 50 battles in He-elzapop
Win 100 battles in He-elzapop
Win 200 battles in He-elzapop
Win 50 battles in Articum
Win 100 battles in Articum
Win 200 battles in Articum
Win 50 battles in Subtyranny
Win 100 battles in Subtyranny
Win 200 battles in Subtyranny
Win 50 battles in Hoarhole
Win 100 battles in Hoarhole
Win 200 battles in Hoarhole
Reach Player Level 10
Reach Player Level 20
Reach Player Level 30
Reach Player Level 40
Reach Player Level 50
Use Fire Elemental Attack 100 times
Use Water Elemental Attack 100 times
Use Wind Elemental Attack 100 times
Use Earth Elemental Attack 100 times
Upgrade one Disciple to Level 15
Upgrade one Disciple to Level 20
Upgrade one Disciple to Level 25