Since you can stake a TINY amount of GAS by holding NEO, when the price of GAS is way higher than the price of NEO people start buying NEO while selling GAS. And the opposite. I believe their price "ratio" (for a lack of better word) will oscillate for a couple of years before it reaches an equilibrium.
RE: Why (and how) you should probably be buying GAS(aka Antcoins aka NeoGas aka NeoCoins) instead of NEO(aka Antshares)