Hi,
you need to generate these files:

META-INF/maven/plugin.xml
META-INF/maven/org.hibernate/enhance-maven-plugin/plugin-help.xml

In maven build system your pom must have
<packaging>maven-plugin</packaging>

Because source code use java 5 annotations for mojo definitions, you must configure maven-maven-plugin to workaround build errors.

<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-plugin-plugin</artifactId>
<version>3.2</version>
<executions>
<execution>
<id>default-descriptor</id>
<goals>
<goal>descriptor</goal>
</goals>
<phase>process-classes</phase>
</execution>
<execution>
<id>help-descriptor</id>
<goals>
<goal>helpmojo</goal>
</goals>
<phase>process-classes</phase>
</execution>
</executions>
</plugin>

Gradle generated pom need correct source and target definition for building in maven build system.

<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.1</version>
<configuration>
<source>1.6</source>
<target>1.6</target>
<encoding>UTF-8</encoding>
</configuration>
</plugin>

and maven-plugin-annotations do not need compile scope

<dependency>
<groupId>org.apache.maven.plugin-tools</groupId>
<artifactId>maven-plugin-annotations</artifactId>
<version>3.2</version>
<scope>provided</scope>

I have modified enhance-maven-plugin.gradle whitch generate correct artifact. (usage : gradle clean pluginDescriptor install)
It was my first experiment with gradle, so you can change everything