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

Last Call (10 points): Successfully complete the Last Call level.
Escape Velocity (10 points): Successfully complete the Escape Velocity level.
Downward Spiral (10 points): Successfully complete the Downward Spiral level.
Rites of Passage (10 points): Successfully complete the Rites Of Passage level.
Second Chances (15 points): Successfully complete the Second Chances level.
All Fall Down (15 points): Successfully complete the All Fall Down level.
Crash Landing (15 points): Successfully complete the Crash Landing level.
Sacrifices (20 points): Successfully complete the Sacrifices level.
There Are Others (20 points): Successfully complete the There Are Others level.
Guiding Fires (20 points): Successfully complete the Guiding Fires level.
The Old Tribes (25 points): Successfully complete The Old Tribes level.
Hidden Agenda (25 points): Successfully complete the Hidden Agenda level.
Jen (25 points): Successfully complete the Jen level.
The Dark Harvester (30 points): Successfully complete The Dark Harvester level.
Following Her (30 points): Successfully complete the Following Her level.
The Complex (30 points): Successfully complete The Complex level.
Ascent (40 points): Successfully complete the Ascent level.
Center Of Gravity (40 points): Successfully complete the Center Of Gravity level.
Resolutions (40 points): Successfully complete the Resolutions level.
Oath Of Vengeance (50 points): Successfully complete the Oath Of Vengeance level.
Facing The Enemy (50 points): Successfully complete the Facing The Enemy level.
Mother's Embrace (60 points): Successfully complete the Mother's Embrace level.
Savior (65points): Successfully complete the game on the Normal difficulty setting.
Galactic Hero (65 points): Successfully complete the game on the Cherokee difficulty setting.
Young Blood (10 points): Get 50 total kills in Death Match.
Brave Star (15 points): Get 125 total kills in Death Match.
Ultimate Warrior (50 points): Get 500 total kills in Death Match.
Team Member (10 points): Get 50 total kills in Team Death Match.
Team Player (15 points): Get 125 total kills in Team Death Match.
Team Leader (20 points): Get 250 total kills in Team Death Match.
Sharpshooter (10 points): Get 25 sniper kills in Death Match and Team Death Match.
Invisible Assassin (10 points): Get 25 kills in Death Match and Team Death Match using spirit form.
Mechanic (10 points): Get 25 kills in Death Match and Team Death Match using the wrench.
Rifle Ranger (10 points): Get 25 kills in Death Match and Team Death Match using the rifle.
Crawler King (10 points): Get 25 kills in Death Match and Team Death Match using Crawler grenades.
Machine Gun Tommy (10 points): Get 25 kills in Death Match and Team Death Match using the Auto-Cannon.
Launcher Lord (10 points): Get 25 kills in Death Match and Team Death Match using the Crawler Launcher.
Soul Sucker (10 points): Get 25 kills in Death Match and Team Death Match using the Leech Gun.
Toxic Overlord (10 points): Get 25 kills in Death Match and Team Death Match using the Acid Sprayer.
Rank 1 (20 points): Win a ranked match.
Ten Ranked Matches (20 points): Play ten ranked matches.

Additionally, there are three secret achievements (with one currently unknown):

Poker Face (10 points): $250 won on Draw Poker.
Retro Gamer (10 points): Score over 15,000 on Runeman.