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

Stranger (75 points): Complete the game on Easy or higher.
Hero (90 points): Complete the game on Normal or higher.
King (100 points): Complete the game on Hard.
Friend of Alexander (5 points): Collect 25 Turtles.
Friend of Ira (5 points): Collect 25 Geodes.
Friend of Judith (5 points): Collect 25 Seeds.
Friend of Douglas (5 points): Collect 25 Honeycombs.
Friend of Bull (5 points): Collect 25 Skulls.
Friend of KW (5 points): Collect 25 Owl Eggs.
Friend of Carol (5 points): Collect 25 Houses.
Veterinarian (10 points): Collect all Turtles.
Geologist (10 points): Collect all Geodes.
Green Thumb (10 points): Collect all Seeds.
Beekeeper (10 points): Collect all Honeycombs.
Hamlet (10 points): Collect all Skulls.
Incubator (10 points): Collect all Owl Eggs.
Real Estate Mogul (10 points): Collect all Houses.
Nova (5 points): Find 5 Stars.
Supernova (10 points): Find 20 Stars.
Star Cluster (10 points): Find 40 Stars.
Galaxy (10 points): Find 65 Stars.
Universe (20 points): Find 95 Stars.
On Fire (10 points): Defeat 250 Fireflies.
Greased Lightning (15 points): Defeat 25 Lightning Bugs.
The Cleaner (20 points): Defeat 100 Shadow Creatures.
Jack of Hearts (10 points): Earn 4 Hearts.
King of Hearts (15 points): Earn 7 Hearts.
Ace of Hearts (30 points): Earn 10 Hearts.
Three Square Meals (20 points): Be eaten by three different Wild Things.
Worst King Ever (10 points): Allow three different Wild Things to fade into oblivion.
Allergen (10 points): Cause three different Wild Things to sneeze.
Gymnast (5 points): Power-swing from three bars.
Demolitionist (10 points): Defeat five enemies with one firefly bomb.
Captain (25 points): Complete a ride with Carol without crashing.
Commodore (25 points): Complete a Sailing mission without crashing.
Glass Houses (10 points): Start a Dirt Clod Fight with multiple Wild Things.
Wild Rumpus! (10 points): Defeat 15 enemies in a row in melee without being hit.

Additionally, there are eleven secret achievements:

Secret achievement (20 points): Unknown.
Secret achievement (20 points): Unknown.
Secret achievement (20 points): Unknown.
Secret achievement (20 points): Unknown.
Secret achievement (30 points): Unknown.
Secret achievement (30 points): Unknown.
Secret achievement (30 points): Unknown.
Secret achievement (40 points): Unknown.
Secret achievement (40 points): Unknown.
Secret achievement (50 points): Unknown.
Secret achievement (50 points): Unknown.