Admiral Ryokugyu has no Bull or Plant Devil Fruit. He is the user of the Energy Paramecia.
In the One Piece world, you can often identify themes for certain groups. Kaido’s crew is about Zoans. His most trusted aids have Ancient Zoans. Big Mom’s crew is about food. The original Admirals were about Logias. What are the current Admirals about? One fruit of each type? I don’t think so. The current theme is science or, to be more precise, the most famous scientist: Einstein.
Vegapunk will soon be revealed. He is the leading World Government scientist. He has a relationship with both Kizaru and Fujitora. He studied Kizaru’s laser and with his new weapon he gave a rationalization for Fujitora to use to get rid of the Warlords. Vegapunk is the One Piece pedant of Einstein.
I don’t think that Oda has deep knowledge of advanced physics. Even if he did, he can’t expect the teenage audience of Jump to have it too. So I won’t argue with anyone here about physics. The One Piece world is not governed by the actual laws of physics.
Most people know that the theory of relativity has something to do with gravity (Fujitora’s devil fruit mind you). His most famous formula is e=mc². c is the speed of light (related to Kizaru’s devil fruit). Mass has something to do wtih gravity plus there is another fruit related to m, one of Crocodile’s people could change her weight. So Ryokugyu will have a fruit related to e, energy.
I imagine the power to take energy from the surroundings. So when someone throws a rock at Ryokugyu, he could absorb the energy. The rock would stop accelerating and just fall to the ground. Ryokugyu could then use the energy to fuel himself. That way he doesn’t need to eat. He can extract all the energy he needs.
I don’t know the full scope of the fruit. I expect it to be very versatile, similar to Moria’s Shadow Fruit and Big Mom’s Soul Fruit.