we changed the logging infrastructure in 5.0.0 to log4j2, and while we did work to mimmic the API, apparently the previous logging framework (Cabin) allowed the logging of an array object, while the new one doesn't (see https://github.com/logstash-plugins/logstash-output-gelf/blob/master/lib/logstash/outputs/gelf.rb#L190)
This should be easy to fix, replacing
@logger.debug(["Sending GELF event", m])
with
@logger.debug("Sending GELF event", :event => m)