$ cd /var/lib/rundeck/libext
$ wget https://raw.githubusercontent.com/tseeker/rundeck-gelf-plugin/master/GelfPlugin.groovy
$ cd /etc/rundeck
# set values as default, since (by bug) framework.properties get not honored
$ grep default /var/lib/rundeck/libext/GelfPlugin.groovy
host defaultValue:"syslog.domain.local", required:true, description: "Hostname to connect to"
port defaultValue:2222, required:true, description: "Port to connect to", type: 'Integer'
$ grep Gelf rundeck-config.properties
rundeck.execution.logs.streamingWriterPlugins=GelfPlugin