You'll have to complete appropriate achievement to get the allotted gamerscore.
Slice & Dice (30) - Kill 20 enemies with the knife.
Sniper (30) - Make 25 headshots.
Soft Touch (30) - Disarm 10 wire traps.
Invisible man (40) - Complete "Frontline" level without killing anyone.
Inquisitor (35) - Kill 2 demons.
Hedge-hopper (30) - On the level "Frontline" kill all of the enemy Red Army and Fascist Soldiers.
Air gunner (30) - Kill 30 enemies using pneumatic weapons.
Cowboy (30) - Kill 30 enemies using revolvers.
Ninja (30) - Kill 10 enemies with throwing knives.
Nosalis hunter (30) - Kill 30 nosalises.
Scrooge (30) - Save 500 military grade rounds.
Rescue Ranger (30) - Save a group of "Reds" from Fascist captivity.
Old school (30) - Kill 30 enemies with the double-barreled shotgun.
Demolitionist (30) - Blow up the tunnel and airlock at "Cursed" station.
DJ Artyom (30) - On the level "Outpost" reach the radio tower and broadcast the commander's message.
Ka-Boom! (30) - Explode 10 enemies.
Merciful (30) - Complete the level "Black Station" without killing any Fascist Soldiers.
Explorer (30) - There is no place in metro you did not visit
Fugitive (30) - Complete level "Armory" without getting caught.
Quick Draw (30) - On the level "Hunter" kill the nosalises before they break through the ventilation grilles.
Quick-witted (30) - Break the support and activate the chandelier in less than 20 seconds.
Raider (30) - On the level "Depot" silently kill the first guard and break into the Fascist station unnoticed.
Ranger (30) - Find all Ranger stashes in Dead City 1 and 2
Wheeler-Dealer (25) - Exchange 500 Military Grade 5.45 rounds at Exchange kiosks.
Fire in the hole (25) - Kill 20 lurkers.
Tank Buster (20) - Destroy fascists' Panzer.
Exorcist (20) - Complete levels "Ghosts" and "Anomaly" without dying.
Metro dweller (20) - A true metro citizen. You know everyone and have seen everything.
Metro Trader (20) - Make 10 deals in weapon shops.
Enlightened (20) - Find the truth.
Pathoanatomist (20) - Kill 5 amoebas.
Heavy Metal (20) - Kill 15 enemies using stationary machine gun.
Heavy Reader (20) - Kill a librarian.
Sherlock (20) - Found all gold ammo, hidden throughout the stations.
Pyro (20) - Kill 5 enemies with a flamethrower.
If it's hostile, you kill it. (20) - Become a true ranger.
Generous (10) - Help the poor, a coin for the kid, medicine for the sick. You help everyone you see.
Realist (10) - A coin for a hungry kid? Get a job.
First blood (5) - What doesn't kill you, makes you stronger.