with the latest Groovy 2.4.6 a Groovy incompatibility with JDK 9 (Compact Strings) https://issues.apache.org/jira/browse/GROOVY-7716 is fixed. Would be nice to have this in Gradle as well, because using Groovy JSON stuff in plugins can break your build (depending on usage of FastStringUtils)…
Full support for JDK 9 in Gradle is planned for the 3.0 release. In the course of implementing the compatibility with JDK 9 it would make sense to also upgrade to Groovy 2.4.6 (or the latest version if possible at that point).