I did some modification to enhance-maven-plugin.gradle.
Maven plugin:descriptor and plugin:helpmojo goals depends on gradle compileJava, so there is no need to compile by maven and for this reason modifying maven-compiler-plugin and maven-plugin-plugin.
Provide dependency on maven-plugin-annotations is fixed.
enhance-maven-plugin build has now standard usage : clean install

There is one issue with this version. There is hardcoded mvn.bat in commandLine, so in linux environment it will be problem.