build.gradle

configurations {
    groovy
}

dependencies {
    groovy localGroovy()
}

project.tasks.create("generateExample", JavaExec) { task ->
  def inputFile = file("input.groovy")
  def outputFile = file("output.txt")

  task.inputs.file(inputFile)
  task.outputs.file(outputFile)

  task.main = "groovy.ui.GroovyMain"
  task.args = [inputFile]
  task.classpath = configurations.groovy

  def output = new FileOutputStream(outputFile)
  task.standardOutput = output
  task.errorOutput = output
}

input.groovy

print "Hello World"