Why does the app go back to dynamic switching when a display is plugged in? The current gpu is also reported in the menu. Otherwise, it will just look like a gear. The gear will have a dot in the middle when the discrete gpu is active. You probably need to allow the application to run in Settings -> Security & Privacy You can also launch it from the terminal and set the desired setting using -integrated, -discrete, and -dynamic.

You can also enable notifications for when your gpu changes (off by default)

The app is simple to control with integrated only, discrete only, and dynamic switching in the menu.