For writing to custom ini file, instead of using one of the globals as file name (GGameIni etc), you need to pass in your file’s absolute path. For instance, if I want to save data in an ini called DefaultMyGame.ini stored in [Project]/Config, I’ll use as path FPaths::ProjectConfigDir() / TEXT("DefaultMyGame.ini").
And for the section, it’s as others said: /Script/YourProjectName.ClassName
If it is a plugin, it’ll be /Script/ModuleName.ClassName