I didn't fish the Coral Coast but when in Fiji (Matava) we encountered GT from about 8kg to 40kg....the vast majority of our fish were in the 15-25kg bracket
Also, it seems a lot of the territory there is reasonably friendly.
I think a PE8 rod will be fine and still good fun on the 20kg fish.
PE6 jig rod might be a bit light, what are you planning on doing with it? If you are just jigging around the reef for reefies, mackerely and stuff it will be fine....there are some seriously big yellowfin and dogtooth though in Fiji and if you are planning on targeting these (I don't know if there are any sea mounts or big drop offs where you are going...) I would suggest a PE8 or PE8-10 rod if you want to land a biggun....Lifting power becomes a real issue with the lighter rods on heavy fish...we landed a couple doggies over 60kg and I really don't know if that would have been possible on most PE6 rods.....chances of coming across that fish might be rare, so the PE6 will probably do for 97% of fish, just the question of whether you are prepared to lose the monster if you find him