Here is the full list of all 27 Gerty achievements.
Take no damage in a single stage
Dodge 8 melee attack with one Dodge Roll
Hit 8 enemies with one Bomb
Try teleporting Grongo to the pit
Complete and survive the game
Reach the secret level
Find the second artifact
Use battery to flee Grongo battle
Spend 5000 Juice in a single run
Complete the game
Defeat Grongo the Slayer
Reach the first boss
Fill all 10 inventory slots
Chain 3 Bomber explosions
Kill the same monster 3 times
Find the first artifact
Fully activate a Monument
Die to a Bomber's explosion
Kill 5 Wormlings with one Mighty Slash
Destroy a Monument
Kill a feared enemy
Kill a Leaper mid-jump with an explosion
Push a monster into a pit
Dodge a bullet with Dodge Roll
Activate all Runes in a stage
Reach Stage 2
Die for the first time