To install phonon-backend-gstreamer-qt5, run the following command in macOS terminal (Applications->Utilities->Terminal)
sudo port install phonon-backend-gstreamer-qt5
To see what files were installed by phonon-backend-gstreamer-qt5, run:
port contents phonon-backend-gstreamer-qt5
To later upgrade phonon-backend-gstreamer-qt5, run:
sudo port selfupdate && sudo port upgrade phonon-backend-gstreamer-qt5
Reporting an issue on MacPorts Trac
The MacPorts Project uses a system called Trac to file tickets to report bugs and enhancement requests.
Though anyone may search Trac for tickets, you must have a GitHub account in order to login to Trac to create tickets.