Here is the full list of all 14 Gemini achievements.
Finish the tutorial
Unlock the Iceworld
Unlock the Desert
Unlock the Underground
Unlock the Underworld
Use the pink character with a pink armor
Use the blue character with a blue armor
Use the cyan character with a cyan armor
Use the yellow character with a yellow armor
Use both equipment drops from the angel enemy
Use both equipment drops from the imp enemy
Achieve 10.000 coins
Get every type of equipment in a single run
Kill more than 1000 enemies