Here is the full list of all 13 GabeN: The Final Decision achievements. It takes around 0-1 hour to unlock all of the achievements on Windows.
Defeat that traitor!
Unlocked by starting a new game.
Congratulations! You beat GabeN: The Final Decision! Feel free to uninstall.
Ok so now you defeated the Illuminati! Congrats! Now you can release Half Life 3! Or can you...
Finally, you defeated the Illuminati! Or did you...
Kill the last admin alive, Wallace Breen.
Destroy the Combine Tank guarding the eastern gate!
Kill GLaDOS and shut down her systems.
Kill the Administrator before she relays her dangerous information to the rest of the admins!
Utterly destroy that illuminati boss!
KIll that commander with your sick burns! (Mixtape not included)
Completely wreck that commander!
Defeat the Xen Leader, Nihilanth!