Maven is a build tool in the Java ecosystem. It needs Maven to build. Maven is bootstrapped in GNU Guix, using generated build configuration files for the Ant build tool, that is already bootstrapped as the result of the From C++ to the world of Java project.
You can download the graph of dependencies from here in a dot format.
Gradle is a build tool in the Java ecosystem. It needs Gradle to build. It also depends on Scala. Scala is currently not bootstrappable, to see the status of work in progress on the issue see Bootstrapping JVM Languages.
It would be nice to check if Gradle also depends on Kotlin, which is another JVM language that is not bootstrapped yet.