Use a brown dwarf, not a gas giant
The magnetic moments of dipoles (reasonable approximations of planetary magnetic fields) fall off with the cube of distance. The strongest known planetary magnetic field, as far as I'm aware, is that of Jupiter: a magnetic moment roughly 20,000 times more powerful than that of the Earth's (control-F "Jupiter’s dynamo").
At the radius of Jupiter, that falls off to roughly 4.71 gauss (bottom of page 5), which is Jupiter's magnetic field strength at its equator. The weakest the Earth's magnetic field gets at its surface is approximately 0.25 gauss. ((4.71-0.3)^(1/3)) ≈ 1.646, meaning that the distance at which Jupiter's magnetic field = 0.3 gauss is 1.646 Jupiter radii away from its equatorial surface. 1.646 Jupiter radii away from Jupiter's equatorial surface is 2.646 Jupiter radii away from its core, or about 190,000 kilometers away from Jupiter's core.
As 190,000 kilometers is well inside the orbital radius of Io, which is slightly larger than the Earth's moon and yet still rendered incredibly volcanically unstable by tidal forces, there's no way an Earth-sized planet is going to survive here — 190,000 km is likely inside the Roche limit for a body that size. And even if it somehow did, the radiation belts would likely render it uninhabitable.
Thing is, though, Jupiter is the planet with the strongest magnetic field discovered so far. Certain brown dwarfs — weird sort of hybrids between ultra-large gas giants and ultra-tiny stars — likely have enormous magnetic fields, on the order of 1,000 times more than that of Jupiter. Given that magnetic moments decrease with the cube of distance, that represents a magnetic field which is ~0.25 gauss at approximately 10x the distance at which Jupiter's is ~0.25 gauss — or, in other words, ~1.9 million kilometers, greater than even Callisto's orbital distance.
Back-of-the-napkin math indicates that:
- the ratio of mass between Jupiter and Io is comparable to the ratio of mass between a particularly large 70-Jupiter mass brown dwarf and an Earth-sized planet
- the ratio of density between Io and Jupiter is greater than the ratio of density between a brown dwarf and an planet with Earth's density
...both of which put together mean the Roche limit for an Earth orbiting a brown dwarf is proportionately less than the Roche limit for an Io orbiting a Jupiter, which means that this planet is less likely to be subject to extreme tidal forces.
In other words, you don't want a gas giant. You want a brown dwarf with an ultra-strong magnetic field and Earth-like planets orbiting it at a radius of over 1% of an AU, where that magnetic field is relatively weak.
Note that the largest brown dwarfs likely glow to a small extent in visible wavelengths (after all, there is fusion going on, just deuterium and lithium fusion rather than hydrogen fusion) and therefore that anyone on these planets will see that, as well as the fact that, within the skies of these planets, it will likely appear comparable to or slightly larger than our Moon.