Where is GameViewportClient.cpp?

I can just find the .h file, but haven’t found the .cpp neither in Unreal Engine folder nor in the folder of the project where I want to change it.

Found it in GitHub folder of UE4. Should it be just there or also in the other mentioned folders?

Actually I’ve been told there where it needs to be. GitHub\UnrealEngine\Engine\Source\Runtime\Engine\Private