The GS Ball is a special item that is required to obtain the Mythical Pokemon, Genesect. Here's how you get it in the game:
* You need to connect to the internet and download the Genesect event. This was a limited-time event, so the event data is likely no longer available.
* After downloading the event, you will receive the GS Ball.
Unfortunately, due to the nature of the event, there is no way to obtain the GS Ball legitimately in the game anymore.
Using cheats or hacks is against the spirit of the game and can potentially corrupt your save file. It's best to stick to legitimate methods of playing Pokemon Black.