The kanji are 月 (Getsu): "Moon", and 光 (Kou): "Light". This is reflected in the weaponskill itself, as the animation illustrates a brightly glowing moon. The moon also has a thin, lime-green, outer circle with Hebrew letters, which translate to, "In the beginning, God created the heavens and the earth.".
Tachi: Gekko is only available to Samurai. While other jobs can obtain the required 225 Great Katana skill they will be unable to use Tachi: Gekko.
Great Katana skill level 225 is obtainable by the following jobs at these corresponding levels: