GoCD has stopped support JDK 6 for some time now. But we understand that some users were using Java 6, so we continued to support it as long as we could while helping users migrate their GoCD servers and agents to Java 7.
Java 6 was declared end-of-life in February 2013, and Java 7 is scheduled to be declared end-of-life soon.
Starting with the 15.1 release of GoCD, it will only run with Java 7. Users are encouraged to upgrade to the latest release of GoCD with Java 8.
Starting with the next release, we plan on providing support for Java 8.