Setting upp integration tests that depends on background process can be a challenge in Gradle. Here is one solution that I have used:

gradle.build

apply plugin: 'java'

/**
* Handle of the background process (script scope)
*/
Process backendProcess

/**
* Task to start the Spring server
*/
task beforeE2eTests {
  ProcessBuilder builder
  builder = new ProcessBuilder('./gradlew bootRun'.split(' '))
  builder.redirectErrorStream(true)

  doLast {
    println "Starting backend"
    backendProcess = builder.start()

    InputStream sto = backendProcess.getInputStream()
    BufferedReader redr = new BufferedReader(new InputStreamReader(sto))

    /**
    * To prevent Gradle to go to next task before the server has started 
    * we add a loop that finds a specific log line. When that 
    * line appears we are good to go to next task
    */
    def line
    while ((line = redr.readLine()) != null) {
      println line
      if (line.contains("Started WebApplication")) {
        println "Backend is ready"
        break;
      }
    }
  }

  finalizedBy 'afterE2eTests'
}

/**
* Task to stop the Spring server
*/
task afterE2eTests {
  doLast {
    println "Stopping backend"
    backendProcess.destroy()
  }

  mustRunAfter 'testAngularE2e'
}

/**
* Task to start E2E tests
*/
task testAngularE2e(type: Exec) {
  mustRunAfter 'beforeE2eTests'

  /**
  * Run the Protractor tests
  */
  commandLine 'node_modules/.bin/protractor', 'e2e/protractor.conf.js'
}

/**
* Main testing task
*/
testAll {
  dependsOn beforeE2eTests, testAngularE2e
}

Tested on OSX 10.15.0 and Gradle 4.10.2