It's several planets. Shattered pieces were flung and formed the asteroid belt due to Jupiter's gravity instead of forming a new planetoid around their neighboring planet. Like our moon for example, was a piece of Earth's crust. There are even Needle like or "cigar" shaped pieces within the asteroid belt that are the original intact pieces of planets, because they usually run into each other and break apart.
The large amount of energy required to shatter or destroy a planet, combined with the asteroid belt’s low combined mass, do not support the hypothesis that the asteroid belt was once a planet. Furthermore, the asteroid belt wouldn’t be a gaiaform. The lore tab for Anteaus Wards implies that every planetoid has a gaiaform, so each individual asteroid would have a gaiaform.