I always figured they'd install a puppet government when and if they succeeded fully. If not, I imagine them just getting all sorts of war reparations, including the Taus and possibly some core Bretonian systems. In that case, it helps the Gaians because, well, look at Kusari proper compared with Bretonia - no smog clouds. And they can continue fighting if need be.
I think that, assuming they succeed, they have enough time to develop better technology before they're at a Bretonian level of crowding. And that may or may not be superior to terraforming, as rumors suggest Planetform's methods may not work long term.