The Magna Martel Group (マグナ・マーテル, Maguna Māteru), or Martel for short, is a worldwide environmental organization based in Naples. This organization is a cover-up and serves as sort of a scientific branch for Gaia.
Martel's origins as an organization are not clear, but it's made very obvious early on that they have enormous power and influence worldwide. With many branches in different countries, including in Kazamatsuri, Japan, Martel carries various kinds of scientific investigation regarding environmental issues. They often offer open conferences with their scientists so people can become aware of how much damage humans do to the planet.
However, the true purpose of Martel is serving as a cover-up for Gaia. Since Gaia is a religious group, and religious cults are not positively regarded in the public eye, they use Martel to spread the message that humans are actually a parasyte on Earth, and that they must be eradicated. This kind of message resonates the best with people who are depressed and wish that their lives would end, the kind of target demographic that would be best suited to become summoners. In this sense, Martel could be considered a recruitment ground for Gaia.