Since there is no way to generate new ones (old rocketry system) and they die of old age, but do not produce offspring, the entire gassy moo population goes extinct after one life cycle (75 cycles). This does not seem intended.
Start a game
Debug mode to reveal all planets
Choose Gasato to view surface
You will need to toggle debug off and then on again to spawn critters
Wait 75 cycles -> All moos die out. You can use debug super speed, this won't change anything about the behavior.