Here is the full list of all 26 Gravi achievements.
Take every path through level 22
Complete any chapter without dying
Complete level 40 without dying
Gather all 40 collectibles
Complete all 10 levels of Chapter 4
Defeat Anti-Gravi once and for all
Complete all 10 levels of Chapter 3
Complete level 30 without dying
Complete level 20 without dying
Complete level 23 without shooting a small shot
Complete level 14 without shooting more than twice
Complete the last puzzle of level 18 wihout touching the middle island
Complete level 18 without retracting a single shot
Complete level 10 in under 37 seconds with the collectible
Complete level 10 without dying
Die from every trap type (Spikes, Teslas, Burners, Lava, Acid, Grinders, and Crushers)
Take the toothy path in level 22
Complete all 10 levels of Chapter 2
Gather 20 collectibles
Get in, get the collectible in level 11, and get out
Complete all 10 levels of Chapter 1
Complete level 5 in under 5 seconds
Die 100 times in one level
Die 100 times
Accumulate a total score over 9000
Gather your first collectible