BNW is not a stand-alone game. You must have the base Civ 5 game to play BNW.
BNW includes all the rules changes of G&K, so if you buy BNW, the only things that G&K and the other DLC adds are Civs, scenarios, and map generators.
If money is tight, I'd suggest buying Civ 5 & BNW and skipping the rest for now. You'll get plenty of enjoyment and play time using the most current feature set from that. If you start getting bored & want more Civs, scenarios, or map generators you can buy DLC and G&K later.
That said, I think the gold edition is well worth the extra $10.
I suppose if you've never played a turn based strategy game, the gold edition without BNW is a cheaper way to see if it's for you, but if you like it, you'll really want BNW.