By default jBPM Business Applications generated via start.jbpm.org are build with Apache Maven. Your generated apps also include build scripts (for Unix, OSX, and Windows) which you can use out of the box to build all the apps modules, as well as launch your app in normal or dev modes.
Using Maven is fine however it excludes Gradle users from being able to build and launch jBPM Business Applications using their favorite build tool.
If you are using Gradle, take a look at this GitHub repo which includes Gradle build files as well as build scripts that call Gradle to build and launch your business application.
Follow the documentation there to set up your Gradle build environment for your generated jBPM business app.
Note that the out-of-the-box Maven build scripts are still the preferred choice when building your app. This is because the limitation of the Gradle scripts not being able to deploy your app on Docker and Openshift. We use the fabric8 plugin to help us do that and this plugin is not currently available for Gradle unfortunately (you can get more info on that here).
Also to note we are not Gradle experts and it would be really helpful if our jBPM community could help us with making the Gradle build for jBPM Business Applications better. If you are interested please clone https://github.com/tsurdilo/jbpm-business-apps-gradle and submit changes via git pull requests and share it to our community. We would really appreciate that!!