Yes, both Ganymede and Callisto have gravity. They are moons of Jupiter, and like all celestial bodies with mass, they exert a gravitational force.

Here's a breakdown:

* Ganymede: It's the largest moon in our solar system and has a stronger gravitational pull than Callisto. Its surface gravity is about 0.146 g, which means someone weighing 100 kg on Earth would weigh about 14.6 kg on Ganymede.

* Callisto: It's the third-largest moon in our solar system and has a weaker gravitational pull than Ganymede. Its surface gravity is about 0.126 g. So, a 100 kg person on Earth would weigh about 12.6 kg on Callisto.

While both moons have gravity, it's significantly weaker than Earth's due to their smaller size and mass.