Answering my own question. The main problem was that I need to run the grails using the full path like this:
Map<String, String> env = System.getenv()
final processBuilder = new ProcessBuilder()
processBuilder.directory(new File("folderFromWhereIWantToRunTheGantScript"))
processBuilder.command([env['GRAILS_HOME']+"/bin/grails","MyOwnScript param1 param2"])
println processBuilder.directory()
Process proc = processBuilder.start()
proc.consumeProcessOutput(out, err)
proc.waitFor()