The Mokujin seems to be only mobile with Hashirama manipulating it using his Chakra. Assuming it has mind of its own and feats shown so far, Mokujin would win easily.
The size of both should be considered as importance because, Mokujin is comparable to 100% Kurama and Perfect Susanoo when it comes to size. Gamabunta is dwarfed easily by size of those creatures meaning, Gamabunta would be so small and can't fight properly.
As offence and defence. Gamabunta's offence are like Katon combined with oil and his Katana skills. Mokujin was capable of tanking multiple slashes of PS and direct TBB of Yin Yang Kurama, so it has quite a lot of defence and they can tank everything Gamabunta throws. Gamabunta's sword is not going to penetrate through it when PS had succeeded after multiple of them.
Even though it lacks feats of offensive attacks, we can logically assume it has a lot of physical strength as it held PS arm. So simply with punch or kick, Gamabunta would be flying.
