I would get 3x4GB. There is no guarantee that your new set will work well with your old set or with all 6 slots occupied. That way, you'll get at least 12GB to use if things doesn't work out and you won't get stuck with 2 sets of 3x2GB and you can only use one set or the other.
I tried this with gaming 7 with G.skill ram, no problem at all. It seems people with problem all have Corsair Vengeance LPX ram, those with G.skill ram are mostly problem free. I think those who want to go with Gigabyte 170 boards should pick G.skill ram for painless experience.
On Gigabyte website, the Gaming 7 is being advertised as Intel Thunderbolt 3 certified motherboard, and there is a thunderbolt driver available in the driver section. If it doesn't have Alpine Ridge, what Intel controller is it using for Thunderbolt?
World Champion <> World record. You can win multiple world championship without breaking the world records as Usian Bolt did.:) No need to argue about it here.