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