The GS ball is a type of pokeball that can not be made by a crafting table and can only be obtained by spawning it in.
In the 3.4.0 version, this ball can be obtained as a boss drop.
It has a 1x catch rate, the same as a normal pokeball, and it's not currently known if it has significance to encounter or obtain any legendary pokemon.
The GS Ball was first seen in Pokemon Crystal Version. It was an event item that could be taken to Ilex Forest to challenge Celebi.