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