The Gamma Case is a CS2 weapon case introduced 7 years ago on 15 June 2016 as part of the "Gamma Exposure" update. The weapon case contains CS2 skins from The Gamma Collection.
The Gamma Case has a chance of being offered to players with prime status once per week as part of their weekly care package. The odds of receiving a case from the rare case drop pool is 1/100 or 1%.
Alternatively, it can be obtained by trading with other players, or purchasing from the Steam Community Marketplace and some third-party marketplaces.