Don't change the mass - change the density.
(Soft science ahead - all hands brace for impact!)
One thing you probably shouldn't do is change Ganymede's mass. That would change its orbit (and its influence on the other moons) in unavoidable and easily observable ways. You'd have to do some elaborate hand-waving to make Ganymede appear to be its apparent mass while having a very different actual mass.
To have a solution from changing the density will still require some hand-waving, but maybe it's allowable in a "cotton-candy-scifi" universe...you can be the judge of that!
To attain earth-like gravity in your caves, we would have to: 1) make Ganymede's core unnaturally dense and its mantle unnaturally light, and 2) place your caves much closer to the core. The handwaving required to make this happen is two-fold:
Firstly, to actually concentrate Ganymede's mass this much in the core, you could not use any naturally occurring material in the known universe. Materials made of conventional elements are too light, and electron- or neutron-degenerate matter would not remain compressed under earthlike gravity--it would explode. 24
So...probably the best soft-sci-fi solution (without invoking artificial gravity generators) is that Ganymede's core contains degenerate matter which for some reason can't decompress. (Is it special matter? Is it in a fluke, naturally occurring statis field? Handwave!) Similarly, you'll need to handwave a material to compose Ganymede's mantle that is extremely light and somehow looks to our telescopes like a salty ocean. (See https://en.wikipedia.org/wiki/Ganymede_(moon)#Composition ) Which bring us to our next point...
We will need to handwave some of our observations of Ganymede's physical appearance and its moment of inertia factor ( https://en.wikipedia.org/wiki/Moment_of_inertia_factor ). To be honest, I don't think there will be any self-consistent and elegant way to explain away all of the observations we've made of it. But at the very least, try to have a reason for why Ganymede's surface is or appears to be made up of water ice and silicate rock, and why it appears to have a subsurface salty ocean and an iron-rich core.
(To tackle the surface, I would offer this...our extremely light mantle-material is somehow also fairly tough and rigid, and the silicate rock of the surface is mostly layers of dust/fragments from meteor impacts.) 70