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

It's HAZARD TIME! (15 points): Start your first game.
Take 5 (5 points): Pause the game for the first time.
Straight-A Student (10 points): Complete all of the in-game tutorials.
Hands-On Approach (10 points): Defeat an enemy using melee combat.
Eat FIST! (25 points): Perform 10 single-hit melee kills.
Feeling Punchy (30 points): Defeat five enemies in a row using melee combat.
Headache (10 points): Score 20 headshots.
Migraine (20 points): Score 50 headshots.
Head of the Class (30 points): Score 100 headshots.
Better than One (25 points): Score two headshots with one bullet.
Plink (10 points): Kill 50 enemies with the Hazard pistol.
Pull! (10 points): Kill 50 enemies with the shotgun.
Auto Dealer (10 points): Kill 50 enemies with the sub-machinegun.
Russian Attack (10 points): Kill 50 enemies with the AK-47.
Peekaboo...I Shot You! (10 points): Kill 50 enemies with the sniper rifle.
Blind Luck (15 points): Kill 25 enemies using blind fire.
Second Amendment (10 points): Kill at least one enemy with every available gun.
Say "No" to Pots (5 points): Shoot 30 potted plants.
Fire Hazard (20 points): Shoot and destroy 30 fire extinguishers.
Uncovered (20 points): Destroy 20 destructible cover objects.
Boom. (20 points): Blow up 50 explosive objects.
No Pain, No Gain (10 points): Kill an enemy with an explosive that also damages you.
Maxx Karnage (20 points): Collect 20 Hazard pickups.
Shield Master (20 points): Collect 20 Master Shield pickups.
Just a Flesh Wound (40 points): Complete a level without dying.
Hazard Avoider (10 points): Complete the game on the Minor Hazard difficulty setting.
Pseudo-Hazard (20 points): Complete the game on the Major Hazard difficulty setting.
Hazard Master (40 points): Complete the game on the Maximum Hazard difficulty setting.
...Where Credit is Due (60 points): Watch the credits from beginning to end.

Additionally, there are twenty one secret achievements:

Secret achievement (30 points): Unknown.
Secret achievement (20 points): Unknown.
Secret achievement (10 points): Unknown.
Secret achievement (10 points): Unknown.
Secret achievement (10 points): Unknown.
Secret achievement (10 points): Unknown.
Secret achievement (5 points): Unknown.
Secret achievement (10 points): Unknown.
Secret achievement (20 points): Unknown.
Secret achievement (20 points): Unknown.
Secret achievement (20 points): Unknown.
Secret achievement (10 points): Unknown.
Secret achievement (10 points): Unknown.
Secret achievement (10 points): Unknown.
Secret achievement (40 points): Unknown.
Secret achievement (20 points): Unknown.
Secret achievement (10 points): Unknown.
Secret achievement (30 points): Unknown.
Secret achievement (15 points): Unknown.
Secret achievement (50 points): Unknown.
Secret achievement (100 points): Unknown.