Here's some pertinent facts:
Apparent magnitudes: Sun from Ganymede = -23.11
Jupiter from Ganymede = -16.02 (full)
Difference = - 7.09
Brightness difference = 2.512 E7.09 = 686 times brighter
Earth comparison: Sun from Earth = -26.74
Moon from Earth = -12.74 (mean full)
Full moon/ Jupiter from Ganymede difference = 20.5 times
Ganymede facts: Orbital period = 1 Ganymede day: 7.16 days
Orbital inclinations: .2 deg to Jupiter equator
2.2 deg to ecliptic
Orbital eccentricity: .0011 (circular)
Orbital speed (ave.): 10.88 km/s
Jupiter facts: Mean radius: about 70000 km
If you are on the "far side" of Ganymede you won't see Jupiter (maybe a glow on the horizon from Ganymede's thin atmosphere). The sun will be 2.512 E3.63 = about 28 times dimmer than seen from Earth (which is still pretty darn bright). A day will last 7.16 Earth days.
The near side will be far more interesting. The sun will go through the same cycle at the same brightness as on the far side. However, Jupiter will "hover" in the sky (maybe a little wobble/ libration) and when full will be about 686 times dimmer than the sun in the sky. Jupiter will go through phases over the course of the 7.16 day cycle and be less bright of course as it approaches eclipse. The eclipse (by rough calculations) will last about three and a half hours. The planet will have an angular diameter of about 7.5 degrees (compare with 0.5 for sun/moon from Earth) and be about 20.5 times brighter than a full moon when Ganymede is between the sun and Jupiter.
A fun side note is that the sky won't be totally dark during eclipse. Light from the sun will reflect off aerosols and hazes in Jupiter's upper atmosphere making Jupiter "glow" in Ganymede's sky.