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.tungsten and uranium deposits resulting in an earth-like overall mass easily 50 times as huge as it should be. Artificial Superheavy elements beyond anything ever manufactured in a lab.
The Surface of Ganymede is covered in a thick layer of 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.
Addendum:
Having gone away and looked up material densities, I realised that the required density in order for Ganymede to be literally 5000 times as massive as it appears is well beyond tungsten or uranium or even Osmium or Hassium.
You need a material with a density of 779,634,464,751.96 kg/m^3 to do it.
I have corrected my answer accordingly.