It's easy to publish sources with the "maven-publish" plugin:

apply plugin: "maven-publish"

task sourceJar(type: Jar) {
  from sourceSets.main.allJava
}

publishing {
    publications {
        mavenJava(MavenPublication) {
          from components.java

          artifact sourceJar {
            classifier "sources"
          }
        }
     }
}

Then go publish to local maven run: gradle publishToMavenLocal

More info in the docs: https://docs.gradle.org/current/userguide/publishing_maven.html#gsc.tab=0

I believe a cleaner and more concise solution for this is to use the maven-plugin, it provides the install task which matches Maven's install task. Update your build.gradle to include the following:

apply plugin: 'maven'

Then to publish (e.g. install) to Maven local repository, run: gradle install or ./gradlew install. See also this solution: Gradle alternate to mvn install

Note: the current Gradle docs (as of v4.9) says the maven-plugin is the "Old Maven Plugin" (but it is not deprecated). https://docs.gradle.org/current/userguide/maven_plugin.html

I think this is the simplest solution to this question at this time.

Perhaps you are better of with a multi-module project. Then the necessity to install doesn't arise at all.

The topic of -sources.jar is discussed here