Good evening everyone. First, i would like to clarify that this isn't a thread based on powerscaling, not saying that the Gorosei and Imu aren't strong.
I've seen lots of theories about Imu being some sort of alien like Kaguya was, or being some sort of primordial entity that's responsible for the creation of the Devil Fruits.
However, this panel heavily suggests that Imu was simply one of the 20 kings responsible for founding the government. This makes me think that Imu, was most likely, a
human.
Imu is probably just a powerful mythical zoan user(a dragon or a demon?) that had the Op Op no Mi immortality surgery performed on him/her.
I've also heard recent speculation that Sabo referring to the Gorosei's powers as "hellish" suggests that they aren't devil fruit users, but instead something else. I highly doubt that's true. The silhouettes also remind me of other zoan silhouettes, like Jabra and Lucci's in Enies Lobby. I doubt they are anything more than mythical zoans or even regular zoans.
This chapter also revived many theories about the Gorosei being some centuries old entities who were present in the Void Century. They could be a few hundred years old(maybe they also had the surgery performed on them?), but they aren't that old. They were literally talking about how the government failed to acquire the Nika fruit after all.
Overall, while Imu and the Gorosei definitely are major characters who are connected to lots of important things in the story's overall lore, evidence seems to suggest that they are just normal humans, not primordial sea gods, not aliens, just humans.
Of course this could easily be proven wrong, but for the time being, I feel like it's heavily implied that they are just normal humans with fruits.