Here is the full list of all 25 Gachi: Space Orgasm achievements.
Complete the first mission successfully
Successfully complete the second mission
Successfully complete the third mission
Collect all statics during the first mission
Collect all statics during the second mission
Collect all statics during the third mission
Collect all weapon improvements during the first mission
Collect all the weapon improvements in the second mission
Collect all the weapon improvements in the third mission
Open all the videos in the flashback gallery (prologue, epilogue, mission rewards)
Kill the first boss on the second level
Kill the second boss on the third level
Kill 10 enemies in the entire game
Kill 15 enemies in the entire game
Kill 20 enemies in the entire game
Kill 25 enemies in the entire game
Kill 30 enemies in the entire game
Successfully complete a mission with a gun
Successfully complete a mission with a machine gun
Successfully complete a mission with a revolver
Successfully complete a mission with a rifle
Make a headshot for the whole game 5 times
Make a headshot for the whole game 10 times
Make a headshot for the whole game 15 times
Make a headshot for the whole game 20 times