scaling Titan/Saturn mass ratio to 5xJupiter mass
I could be screwing up, but this site (http://cseligman.com/text/moons/saturn/titan.htm) gives Titan's mass as .0225 x earth's mass. If the Titan/Saturn mass ratio is not unusual, then with Jupiter about three times Saturn's mass, and a 5-Jupiter mass giant could have 15 x .0225 earth mass satellite, or .3375 earth mass. Don't know if that's big enough to be interesting in a star's HZ.
Getting back to HD 188015b, the gas giant is 1.26 x Jupiter's mass. Our very limited data does not bode well for the chance of having an earth-sized moon there.