Invalid option '-f' in systemd unit file
The osmo-ggsn.service file  contains the '-f' switch, which is not a valid option (anymore?).
In dda21ed7d4a897c9284c69175d0da598598eae40 from August 2017, osmo-ggsn getopt parsing was changed from a
custom command line parser (called "cmdline_parser") to standard getopt. The old parser supported the
--fg option ("run in foreground"), and -f was an alias for --fg.
In the same commit a new '--daemonize' option was introduced which enables background execution and defaults to off.
So the former behaviour with -f is now the default behaviour.
It looks like commit dda21ed7d4a897c9284c69175d0da598598eae40 simply overlooked the systemd config file.