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

Reporting for Duty (25 points): Kill 250 enemies.
More Than Enough (50 points): Kill 500 enemies.
Tough Guy (75 points): Kill 750 enemies.
Sprint to the Finish (20 points): Complete a mission below the target time.
Minuteman Medal (50 points): Complete all missions below the target time.
Overachiever (50 points): Complete a mission below the target time and completed all the bonus objectives.
Sniper (5 points): Killed 75 enemies with head shots.
Close Combat (20 points): Kill 25 enemies using melee attacks.
Raining Grenades (20 points): Kill 50 enemies using grenades.
Quick Draw (20 points): Kill 50 enemies using sidearms.
Cannons Away (20 points): Kill 25 enemies using cannons.
Gat-slinger (20 points): Kill 50 enemies using Gatling Guns.
Demolitions (50 points): Shoot all explosive barrels in every mission.
Marksman (10 points): Complete a mission with over 75% accuracy.
Bullseye (30 points): Complete a mission with over 95% accuracy.
Pioneer (10 points): Earn the rank of Pioneer.
Corporal (15 points): Earn the rank of Corporal.
Sergeant (20 points): Earn the rank of Sergeant.
Lieutenant (25 points): Earn the rank of Lieutenant.
Captain (30 points): Earn the rank of Captain.
Major (35 points): Earn the rank of Major.
Colonel (40 points): Earn the rank of Colonel.
General (50 points): Earn the rank of General.
Attention to Detail (5 points): Complete one bonus objective.
To the Letter (40 points): Complete all bonus objectives.
Chancellorsville (10 points): Complete the Dawn at Chancellorsville mission.
Cold Harbor (10 points): Complete Dr.Gatling`s Secret Weapon mission.
Hampshire (10 points): Complete The Gray Ghost mission.
Gettysburg (10 points): Complete the Sharpshooters at Devil`s Den mission.
Martinsburg (10 points): Complete The Great Train Raid of 1861 mission.
Grand Gulf (10 points): Complete the USS Pittsburgh mission.
Jackson (10 points): Complete the Infiltration of Jackson mission.
Lynchburg (10 points): Complete the Sabotage at Lynchburg mission.
Vicksburg (10 points): Complete the Captain Blazer`s Scouts mission.
Vicksburg Gate (10 points): Complete The Fall of Vicksburg mission.
Ironclad (10 points): Complete the Siege of Vicksburg mission.
War Survivor (20 points): Complete the game on the easiest difficulty setting (or higher).
War Veteran (40 points): Complete the game on the medium difficulty setting (or higher).
War Hero (75 points): Complete the game on the hardest difficulty setting.

Additionally, there is one secret achievement, which is currently unknown:

Secret achievement (20 points): Unknown.