"The gas giant would also have to be inside the solar system's habitable zone, which, as I understand it, may not be possible. I wouldn't mind reading some detailed science papers on this subject."

Not necessarily - besides the concept of "habitable zone" is being redefined. Conditions within Europa and other Jovian (and perhaps Saturnian) large satellites could provide all of the ingredients we consider necessary for life. Several km of ice is a nice radiation shield too. Dozens of klicks is even better. As such they reside in a "habitable zone" - one created by Jupiter (or other large world). In this case the main energy pumped into them is mechanical via tidal stretching of their innards whereas on Earth it is radiant. Primary producers are photosynthetic autotrophs in those parts of the ecosphere where light is bountiful yet primary proucers are chemoautotrophs at depths where light is absent (although IR lightfrom heat sources may pump some photosynthetic systems in bacteria) If you look at life in the abyssal depths of terrestrial oceans everything there - energy, nutirents, is the direct result of things seeping out of Earth's hot interior.