The Gradle plugin has the following documentation for the "Root Build script" attribute:

If your workspace has the top-level build.gradle in somewhere other than the module root directory, specify the path (relative to the module root) here, such as ${workspace}/parent/ instead of just ${workspace}. If left empty, defaults to build.gradle

The doc is confusing because:
1) The doc states the path is relative to the module root, but at the same time it provides an example with an absolute path - assuming ${workspace} is expanded to an absolute path.
2) The doc provides ${workspace} as an example, is this referring to the WORKSPACE environment variable? Then shouldn't it be all caps?
3) It's not clear if the user is supposed to enter a directory name or a file name. The documentation has an example with ${workspace}/parent/ which is a directory, but the doc also states the default is build.gradle, which is a file.