Gradle 2.5 aims to support JDK 1.9.
In fact, the Gradle 2.5 nightly build already supports building spring-test (and presumably the rest of the spring-framework) against JDK 1.9.
For details, see the comments from Adrian Kelly on the Gradle Forums.
To be clear, it's not quite guaranteed that 2.5 works flawlessly with Java 9. We are making steps towards this; how far we get remains to be seen.
At this time we won't be doing a point release of 2.4. You could try using the http://gradle.org/nightly2 build in the interim.
We are looking to issue the first RC of 2.5 in the next two-ish weeks.