Today I've released the patch 1.2.6 of logstash-gelf. This release contains a performance bugfix which caused an incident in the past.
The java.util.logging hoghandler's publish method was synchronized. This caused stalling of different threads which ended up in a denial of service. Usually a restart of the application is sufficient to solve that problem, but that's only a temporary solution. Other implementations of logstash-gelf (Log4j, Log4j2 and Logback) are not affected.
Any feedback is appreciated or create issues on GitHub.