Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

Start with the Corners (15 points): Found your first puzzle piece.
Puzzle of Flesh (15 points): Found 1/4 of the puzzle pieces.
Starting to come together (15 points): Found 1/2 of the puzzle pieces.
Putting it all together (30 points): Found all the puzzle pieces.
Paper-Mache (15 points): Found your first Billy Puppet.
Puppeteer (30 points): Found all 7 Billy Puppets.
A Little Background (15 points): Found your first Case File.
Some light reading (15 points): Found 1/4 of the Case Files.
Half the story (15 points): Found 1/2 of the Case Files.
Journalist (30 points): Found all the Case Files.
Play ME! (15 points): Found your first Audio Tape.
Mix Tape (15 points): Found 1/4 of the Audio Tapes.
Easy Listening (15 points): Found 1/2 of the Audio Tapes.
Reel to Real (30 points): Found all of the Audio Tapes.
Router (15 points): Beat the best time for the Point to Point Minigame (Normal).
P2P (15 points): Beat the best time for the Point to Point Minigame (Insane).
Electrician (15 points): Beat the best time for the Polarized Circuit Minigame (Normal).
Jury Rig (15 points): Beat the best time for the Polarized Circuit Minigame (Insane).
Chromatic (15 points): Beat the best time for the Kaleidoscope Minigame (Normal).
Thinking Three Steps Ahead (15 points): Beat the best time for the Kaleidoscope Minigame (Insane).
Gear Grinder (15 points): Beat the best time for the Jigsaw Box Minigame (Normal).
Mechanic (15 points): Beat the best time for the Jigsaw Box Minigame (Insane).
Running like Clockwork (15 points): Beat the best time for the Weapon Case Minigame (Normal).
Second Amendment (15 points): Beat the best time for the Weapon Case Minigame (Insane).
Tumbler (15 points): Beat the best time for the Dial Puzzle Minigame (Normal).
Forced Perspective (15 points): Beat the best time for the Dial Puzzle Minigame (Insane).
Connect the dots... (15 points): Beat the best time for the Lights Out Minigame (Normal).
Burned Out (15 points): Beat the best time for the Lights Out Minigame (Insane).
Smooth Criminal (15 points): Beat the best time for the Lockpick Minigame (Normal).
Master Thief (15 points): Beat the best time for the Lockpick Minigame (Insane).

Additionally, there are 20 secret achievements:

Sarah Blalock (30 points): Freed Sarah From The Trap.
Joseph Poltzer (30 points): Freed Joseph From The Trap.
Carla Song (30 points): Freed Carla From The Trap.
Solomon Bates (30 points): Freed Solomon From The Trap.
Henry Jacobs (30 points): Freed Henry From The Trap.
Would You Like To Play A Game? (15 points): Completed The Eye Surgery Minigame.
Compassion (15 points): Saved A Person That You Didn't Have To.
Did I Fire 6 Shots Or Only 5... (15 points): Joseph Died.
Firefighter (15 points): Barry Died.
Hands Off (15 points): Zeke Died.
Third Time's The Charm (15 points): Subdued "The Bullish Man".
Hedgehog's Dilemma (15 points): Donovan Died.
Brought To You By... (15 points): Watch the credits without skipping.
You Wasted Your Life (15 points): Played on Christmas.
It Must Be SAW (15 points): Played on Halloween.
Old School (15 points): Entered The Legendary Konami Code.
Path Of Blood (75 points): Campbell Survived His Test.
Path Of Flesh (70 points): Campbell Did Not Survive His Test.
Battering Ram (15 points): Defeated "The Bullish Man".
Now THAT'S Using Your Head! (15 points): Escaped "The Bullish Man".