Shiki ate the Fuwa Fuwa no Mi, a Paramecia type Devil Fruit which allows him to make surrounding non-living matter levitate once he has come in contact with it.
Shiki states that once he makes something float it will stay that way until he himself undoes it or he loses consciousness. He has been shown to make several Marine vessels and even vast island areas float.
Shiki seems to have mastered the use of this fruit, as he has shown a variety of skills which manipulates the surrounding non-living environment in complicated ways, even manipulating it from its simplest components, such as earth’s fragments and snowflakes being used to build up gigantic, elaborate structures that are then used to attack.
Fujitora ate the Zushi Zushi no Mi, a Paramecia-type Devil Fruit that allows him to manipulate gravity around a chosen location at will by sending gravitational forces.
He regularly uses this ability to precipitate giant meteorites from the sky or, as shown at Green Bit, uses it to increase the force of gravity and immobilize an opponent on the ground.
Fujitora can also reverse gravity to create a circular barrier around him that repels all attacks or to levitate a battleship similar to Shiki’s Fuwa Fuwa no Mi. He can also levitate a rock and use it as a mobile platform.
So both Shiki and Fujitora’s devil fruits can manipulate gravity.
The Difference is that while Fujitora increases gravity, Shiki negates gravity.
They also are literally and metaphorically opposite: