Professor Elm does not offer a GS ball in "SoulSilver," nor is there any way to naturally acquire the ball in this version of Pokemon. The GS ball can only be acquired by hacking the game with a third-party device, such as an Action Replay.
The mysterious GS ball is only found in the Japanese version of "Pokemon Crystal," and as of the sixth generation, is not found in any English version. The ball is used to trigger an event in which the player can capture the ultra-rare Pokemon, Celebi. Because of this, there is no native way to acquire either the GS ball, or Celebi, without hacking the game.