Except for the fact that it would be tidelocked at any reasonable distance from the gas giant. Problem - to have an earth-sized moon, you need a massive gas giant (about 5-10 jupiter masses). More massive gas giants tend to have very strong magnetic fields. Strong magnetic fields close to stars (i.e. in the habitable zone) trap and accelerate lots more solar wind particles than in gas giants further away (so if Jupiter is bad, a more massive jovian closer to the star should be worse). Maybe an earth-sized rocky/metallic moon could generate a strong enough magnetic field to keep those charged particles at bay, but it's going to be a struggle.
If we put the moon further away then we have more chance of being outside the magnetic field. But there's only so far you can move the moon out before it gets into unstable orbits because the Hill Sphere (the sphere of influence) of the superjovian is going to be pretty small that close to the star. But if we can put the moon outside the jovian's magnetosphere, we run into another issue - it's still (probably) going to be tidelocked to the planet, and being further away that means that it's day (relative to the sun) is going to be more like a week or two long. Also, solar tides could mess with that (I'm not sure if they can or not - but the interplay between planetary and solar tides at that distance may mean that the moon gets into a weird resonance which could make the rotation period even longer, like one rotation every three orbits or something).
So the upshot is... it's pretty complicated ;).
If we put the moon further away then we have more chance of being outside the magnetic field. But there's only so far you can move the moon out before it gets into unstable orbits because the Hill Sphere (the sphere of influence) of the superjovian is going to be pretty small that close to the star. But if we can put the moon outside the jovian's magnetosphere, we run into another issue - it's still (probably) going to be tidelocked to the planet, and being further away that means that it's day (relative to the sun) is going to be more like a week or two long. Also, solar tides could mess with that (I'm not sure if they can or not - but the interplay between planetary and solar tides at that distance may mean that the moon gets into a weird resonance which could make the rotation period even longer, like one rotation every three orbits or something).
So the upshot is... it's pretty complicated ;).