To unlock the Completionist trophy in Borderlands: Game of the Year Edition, you will need to complete all 43 missions available in The Secret Armory of General Knoxx DLC.
The missions are below:
Wanted: Dead!
Armory Assault
Bridging the Gap
Athena Set Up Us The Bomb
Knoxxed Out
This Bitch is Payback
This Bitch is Payback, pt. 2
Loot Larceny
Rendezvous
Code Breaker: Analysis
Scooter? But I Don't Even Know Her
Circle of Duty: New Recruit
Circle of Duty: Cadet
Circle of Duty: Private
Circle of Duty: Corporal
Circle of Duty: Sergeant
Circle of Duty: Medal of Duty
Claptrap Rescue: Lockdown Palace
Code Breaker: Time is Bullets
Core Collection
Mop Up
Super-Marcus Sweep
Local Trouble
It's Like Christmas!
Prison Break: Road Warrior
Prison Break: Over the Wall
Prison Break: Try Not to Get Shanked
Thrown for a Loop
Stain Removal
Lost Lewts
Boost the Monster
Greasemonkey
You've Got Moxxi: Roadblock
You've Got Moxxi: Moxxi's Red Light
OMG APC
Big Crimson Brother is Watching
Bugged
Drifter Lifter
Power Leech
Road Rage
You. Will. Die.
Purple Juice!
Little People, Big Experiments