Jack, what you're looking for unfortunately doesn't exist and the idea of focussing for big GTs on small lures is a bit counter-intuitive.
A rod that can handle 15kgs of drag will be a rod that is rated around PE10-12. I know of no PE6 rod that is designed to handle drag pressure of that magnitude. PE6 rods generally operate around a maximum level of 10kg.
Big GTs will take small lures but people who focus on catching these generally use big poppers. I don't know why you're thinking of trying to focus on catching big GTs on small lures
Also, if you're wanting to catch big GTs, I really don't think PE6 is the way to go.