GSC, also known as "game script code", is Call of Duty's scripting language that is pretty much based off C++. The syntax is exactly the same, and the language is very limited. It is only ever used when a game/map is running.
Where do I start?#
You can start by reading our How to: GSC guide.
Fortunately, the Plutonium community has various Modding Support & Discussion categories for each game, and various Discord channels, there you can ask for help or help others, and discuss modding-related topics.