There's been a big shift in the mechanics of gold trading lately, observes Dennis Gartman. For the past four to five years, the spot price of gold has risen while gold miners have languished, but no more. Recently, gold mining stocks have outperformed the advance in the yellow metal, which Gartman sees as a positive sign that the rally in spot gold may have legs. How to play it now? He recommends buying the Market Vectors Gold Miners ETF (GDX).