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

Still Creek Savior (20 points): Save all the survivors in Still Creek.
We Ride To Fortune City! (20 points): Escape Still Creek with Katey.
Chop Shop (20 points): Defeat Jed the Mechanic.
Zombie Hunter (10 points): Kill 100 zombies.
Zombie Exterminator (20 points): Kill 1,000 zombies.
Small Town, Deep Pockets (20 points): Spend $100,000 in the pawnshop.
Part Way There (10 points): Find and return a bike part.
Ready to Ride (20 points): Rebuild the bike.
A Taste of Things to Come (10 points): Build a combo weapon.
A bigger taste... (20 points): Build all available combo weapons.
Locksmith (20 points): Unlock all the doors in Still Creek.