Ganymede itself is packed with high-density materials such as tungsten and uranium deposits resulting in an earth-like overall mass easily 50 times as huge as it should be.

The Surface of Ganymede is covered in a thick layer of [Cavorite](http://www.yourdictionary.com/cavorite) Dust, resulting in its unusually high density being almost wholly cancelled out, what gravity/Mass-Effect that filters through the Cavorite is only a couple percent of its natural strength.

Within the caves, gravity is unaffected and the inhabitants experience earth-normal conditions.

If you want the surface itself to have earth-normal gravity, you could handwave that the Cavorite attenuates the effects of gravity so that it falls off very rapidly, eg: over a matter of meters. Shortening the length of the gravity waves to something you could measure on a yard-stick. Meaning you can walk around as normal, but throw a ball high into the air and it won't be coming down again.