Hi, Mark.

I'll provide some interesting
ratios, here.
Jupiter is 12,000 times more massive
than it's largest/most-massive moon, Ganymede.

If that ratio holds for giant-planets to their moons, then an earth-mass 'moon' would have to orbit a "brown dwarf" [with a mass 40 times that of Jupiter].

But there may be a more optimistic planet-mass to moon-mass ratio
[ignoring our own Earth - Luna system].

Saturn is about 4000 times more massive than Titan; coincidently,
Neptune almost has the same mass-ratio
over its moon Triton.
If that ratio were to apply to those
super gas-giants orbiting other stars,
than a gas-giant with 12-14 times the
mass of Jupiter will do for it possess
an earth-mass 'moon'.

And, by the way, since gas-giants
with four to five-times the mass of Jupiter have been located orbiting
stars like "70 Virginis", etc., then
Mars-mass moons may orbit such
giant planets if the 12,000-to-one
"gas-giant to moon mass-ratio" is applicable...and I think it is.

cheers.

Jacob