Getting hold of ApplicationContext and grailsApplication is possible though bootstrap command. Include _GrailsBootstrap script, then call configureApp () or depend on it in order to make ApplicationContext available in script:

includeTargets << grailsScript("_GrailsInit")
includeTargets << grailsScript("_GrailsBootstrap")

target(loadGames: "The description of the script goes here!") {
   depends(configureApp)

   def listFile = new File('list.txt')

   listFile.eachLine {
      //Once configureApp() called ApplicationContext can be accessed as appCtx
      def result = appCtx.getBean("bggService").search(it)
      println it + " " + result.length()
   }
}

setDefaultTarget(loadGames)