But how close does Gradle 3.4 get us?

I still personally have not seen any official word about proper Java 9 support in Gradle. The most I have heard is:

{quote)
(from https://discuss.gradle.org/t/java-9-build-9-ea-155-mixed-mode-problems/21618)

We currently don't have capacity to look into these problems, especially since Java 9 proved to be so much of a moving target. We will start working on Java 9 support again in April.