create version.txt file in project dir via build.gradle task

The example you’re referring to is almost correct. With a couple of minor tweaks it works as expected:

import java.text.SimpleDateFormat
import org.ajoberstar.grgit.Grgit

plugins {
    id "org.ajoberstar.grgit" version "1.7.2"
}

version = 1.0

task versionTxt()  {
    doLast {
        new File(projectDir, "version.txt").text = """
Version: $version
Revision: ${grgit.head().abbreviatedId}
Buildtime: ${new SimpleDateFormat("dd-MM-yyyy HH:mm:ss").format(new Date())}
Application-name: foobarbaz app
"""
    }
}

Run gradle versionTxt to get the desired output.