Gateron X's take the classic Gateron Yellow and make it smoother due to its factory polishing, as seen in Gateron CJ's. These are slightly more expensive then gateron yellows, and use the Gateron KS-3X47 mold. You can tell if it's an X switch or not by looking at it's middle post, it'll have a X.
Like gateron yellows, these are decently scratchy but much smoother due to the polishing. 205g0 will make it sound better, but feel worse.
105g0 will make it sound slightly better, and feel better.