Personally I'd stick to the Honda as the last thing you want on soft paint is brackets.
Yep they aren't cheap but they do fit the GT.
A long board should be no problem.
I've been carrying a 7'6" board on mine although I had a paint problem it was down to faulty spraying at the factory and not any other causes, a very rare occurrence.
I believe from memory you can carry up to 30KGs on the standard rack.
BTW the Honda fittings are there, they're just under plastic roof trim strip - you're dealer will cut it out when fitting the rack and supply clip on covers for when its off.