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