Gaians originated from disgruntled Bretonian researchers, and established themselves to protect the Planet Gaia, which is in Bretonia. People from elsewhere might agree with the eco-friendly ideals, but I doubt many care about a single planet in Bretonia. They also get the majority of their recruits from Bretonia. Bretonia, unlike Kusari and Rheinland, is creating a lot of waste (See London Debris fields, Leeds). This is what causes those eco-activists to be terrorists, and not just a political movement for more eco-friendly industry. Liberty has a good portion of waste as well in Texas and New York, so some Gaians having been Libertonians makes sense as well.
However, Rheinland and Kusari are both fairly clean in comparison to the other houses. Kusari especially. There is little reason for Kusarians to be itching to go join the Gaians, since they haven't had to experience the kind of harmful environment the Liberty/Bretonia industry creates. The only ones who would would be extremists, which is an extremely small number of people. Contrary to popular belief, Gaians aren't all extremists. Few are actually. They became terrorists because their political battles were getting nowhere. Making the Gaians was a desperate act, not an extremist one.
So yes, there are probably Gaians from the other three houses as well, mostly Liberty. But the majority of Gaians are still Bretonians.
EDIT: @Ritsuka; The Corsair-Gaian alliance is only shown on the repsheet, because it was not supported by any lore in the game, vanilla or mod. This is why so many people had issues about the Corsair-Gaian alliance, because it was unsupported and made very little sense. There is as much reason for Gaians to be enemies of Corsairs as there are for them to be friends.