His Spirit is different from Gu Yue. Gu Yue's Spirit is a 7-Colored Dragon with each color representing the Elements she control and the colors aren't really that of a rainbow while Gongyang Mo's Spirit is a Rainbow Dragon. It's colors are those of the Rainbow as well as representing Support Abilities he possess. At the same time, he could only Cultivate until Rank 70(Spirit Sage/Saint & 7-Rings) due to the limitations of his Spirit. It's the same with 7-Treasured Glazed Tile Pagoda's problem of not being able to cultivate pass Rank 70 because the pagoda only has 7 floors. On Gongyang Mo's case, he only has 7 colors of the Rainbow so he won't be able to cultivate pass Rank 70. Gu Yue is different from him as she could cultivate up to Title Douluo since the 7 Colors of the Dragon that she possess isn't a limitation rather a representation of the Elements she possessed.