Well, BNW does come with the gameplay elements of GnK (like religions and espionage), and you don't need GnK for it to run. You just miss out on most of the GNK civs (except Ethiopia, and maybe something else). But...it appears that the Gold Edition Upgrade is cheaper than BNW. So...yeah. The Gold Edition Upgrade gives you a lot of content, including a bunch of fun DLC civs and GNK, but then you'd miss out on the stuff from BNW.
Anyway, it sort of depends how much the BNW stuff is worth to you. I enjoy the new elements of BNW. The ideologies and trade routes are a lot of fun, and so are the new civs.
