Not sure if it was the same problem, in version 4.8.1, I created my GameViewportClient cpp subclass and set it in the project settings, but I was getting the error
“Engine config value GameViewportClientClassName is not a valid class …”
To solve this I edited DefaultEngine.ini and set the path of GameViewportClientClassName without the /Script/ part, like this
GameViewportClientClassName=MyProject.MyGameViewportClient
Now the class is being used correctly and no error is shown.