Gastromo is a slug-like monster who possesses anyone that it can find, and reacts to salt the same way real slugs and snails do. Once Gastromo finds its host, it jumps inside the host's body turning him/her into a slug monster. This causes the host to have a large craving of anything containing high amounts of iron.
Describing his appearance, Gastromo is a fish-like monster with mahogany-brown scales, six blood-red alien-like eyes, and hand-like fins.
Role in the episode
Gastromo possessed Martin and the possessed Martin made M.O.M. and Java into slave slug monsters. Diana defeated Gastromo on her own by using an electromagnet to pick up Gastro-M.O.M. and his slaves (who contained high amounts of magnetic iron) and dropped them into a salt-water fish tank. This caused Gastromo to separate from Martin and free Java and M.O.M. from their "slugification". Then the two fish ate Gastromo, and M.O.M. had the tank sealed so she could keep an eye on Gastromo.
- Gastromo was the first monster that M.O.M. caught when she was an agent.