Hi Jezza,
Saw a few GT's caught on these out in the Coral sea. Fast sink version has a very nice subtle swimming action - just cast it out and wind it in / stop-start. Not sure it would be the first lure out of the bag - but might be worth a go if the GT's aren't quite on the chew.
Best rigged with a baker or treble on assist cord (or swivels & spilt rings) attached to your leader swivel so the hook is under the belly of the head segment of the lure. You will have to rig this ways as there's no way the wiring of the lure will stand up to even a small GT (I don't think they were designed as a GT lure).
Hope this helps.
Dave