Let's say you wanted to install this plugin: https://github.com/finnlabs/openproject-meeting. Here is how you would do it with the openproject version packaged at https://pkgr.io/apps/tessi/openproject:
cat >> /opt/openproject/Gemfile.plugins <<EOF
gem "openproject-plugins", :git => "https://github.com/opf/openproject-plugins.git", :branch => "stable"
gem "openproject-meeting", :git => "https://github.com/finnlabs/openproject-meeting.git", :branch => "stable"
EOF