Here is the full list of all 10 Gravitron 2 achievements.
Complete the entire game.
Rescue 100 scientists.
High shots fired to kill ratio. Must destroy atleast 20 enemies.
Destroy 20 fuel pods.
Complete any stage over 5 without hitting anything.
Complete any stage over 5 without destroying any enemies.
Complete any stage over 5 in under 1 minute.
Destroy 250 enemies.
Guide a missile into an enemy.
Destroy an enemy with a reflection.