Here is the full list of all 21 Gravity Island achievements.
Gather 10 Lumies
Gather 1 Lumie
Complete 3 levels
Complete the first level
Activate the first gravitational field
Use the jump ability for the first time
Hit an unexpected obstacle
A flaming arrow has you in its sights
Sometimes things blow up
Use the teleporter for the first time
Gather 30 Lumies
Gather 100 Lumies
Gather 200 Lumies
Gather all of the Lumies
Complete all levels in the Jungle World
Complete all levels in the Ice World
Complete all levels in the Fire World
Complete all levels in the Dream World
Complete 10 levels
Complete all levels
Gather all of the Lumies. Complete all levels