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