There are Gts in the waters around Nadi, including some surprisingly close to Denarau, but unfortunately the local charters kill everything and their casting gear is rubbish (I'm happy to be proved wrong on both counts!)
I would see if you can wangle a day either down at Pacific Harbour with Callan at Xstreem Fishing Charters or up at Volivoli with Nick on Half Cast. Both these guys are forum members, have great boats set up for casting, and offer good quality hire gear.
If you can't make it away from Denarau, then talk to the operators there and tell them exactly what you want to do, emphasising releasing any Gts. Beware of them linking you up with other "anglers" as you'll end up trolling for hours! Make sure you bring your own gear.
Best of luck,
Will