The doc at Gatling - Installation states that “All maven, gradle and sbt demo projects contain some helper classes you can use to trigger some Gatling tasks”. In fact I couldn’t find the Engine class in GitHub - gatling/gatling-gradle-plugin-demo-java.
After some search I found it in gatling-maven-plugin-demo-java.
In addition, the main method of the Engine class should return the int return value of the fromMap call.