Shadow Sky is a Shadow version of Hail and Sandstorm, and damages all non-Shadow Pokémon at the end of each turn by 1/16 of their maximum HP. It also changes the type of Weather Ball to ???-type.
As Shadow Pokémon cannot be traded, and since Pokémon Contests do not exist in Pokémon XD: Gale of Darkness, Shadow Sky cannot be used in Contests.
Shadow Sky is only available in Pokemon XD: Gale of Darkness, and is not seen in any main games.
Also, it's spelt Ghetsis (took me so long to remember this).