Gassy Moos are supposed to eat Gas Grass. However, they do not, at all. It doesn't matter if the Gas Grass is tame or wild, they won't eat, and thus won't produce Natural Gas.
This makes them totally useless, and also makes the farming of gas grass totally useless as well.
I only tested in sandbox with wild Gassy Moos, however another bug report mention they don't eat when domesticated as well:
Here is a test sandbox save showing the issue:
- Fullfill Gas Grass growth requirements
- Put a Gassy Moo in the room
- Gassy Moo stays hungry despite perfectly good Gas Grass available