Hey mate,
I had the same dilemma a couple of months ago, was over in Airlie Beach for my brother-in-laws wedding, was trying to hook up a charter but never managed to get anything sorted so didn't get out.
I did find these guys who look pretty good:
I went in to one of the local tackle stores and had a yarn to the old dude who owned it, he recommended them as well. They seem to keep their facebook page updated with availability so maybe keep an eye on that.
Depends where your staying but theres apparently a fishing pontoon down at Shute Harbour that people apparently have caught GT's off, but I don't know any more details than that. But you'd need your own gear.
Like I say, I never got out fishing when I was there so thats all I can really tell ya. I didn't really look into doing any longer multi-day charters.
I'd be interested to know how you get on.