Here is the full list of all 12 Osmos (GFWL) achievements worth 200 gamerscore.
Complete all tutorial zones.
Unlock all zones.
Collapse the system in level F2-3 by feeding the Attractor.
Complete all Ambient zones.
Complete all Force zones.
Complete all Sentient zones.
Complete the game.
Complete three levels in each Ambient Forever zone.
Complete three levels in each Force Forever zone.
Complete three levels in each Sentient Forever zone.
Complete five levels in any Forever zone