To convert Maven to Gradle, the only step is to run gradle init  in the directory containing the POM. This will convert the Maven build to a Gradle build, generating a settings.gradle file and one or more build.gradle files. 

That's all!

To convert Gradle to Maven, first, Maven plugin need to be added in the build.gradle file. 

build.gradle should look like this: 

apply plugin: 'java'
apply plugin: 'maven'
 
group = 'com.bazlur.app'
// artifactId is taken by default, from folder name
version = '0.1-SNAPSHOT'
 
dependencies {
compile 'commons-lang:commons-lang:2.3'
}

Then, simply run Gradle install in the directory.

Lastly, simply run gradle install and the directory containing build.gradle will do the job. It will create pom-default.xml in the build/poms subfolder.