To install gtksourceview5, run the following command in macOS terminal (Applications->Utilities->Terminal)
sudo port install gtksourceview5
To see what files were installed by gtksourceview5, run:
port contents gtksourceview5
To later upgrade gtksourceview5, run:
sudo port selfupdate && sudo port upgrade gtksourceview5
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.