Code-08.gs

function labelMessagesAsDone(messages) {
  var labelName = 'scripting-emails';
  Logger.log('Processing labels...');
  var mylabel = GmailApp.getUserLabelByName(labelName);

  if (!mylabel) {
    mylabel = GmailApp.createLabel(labelName);
    Logger.log("Label doesn't exist, creating new " + labelName); 
  }

  for (var m = 0; m < messages.length; m++) {
    mylabel.addToThread(messages[m].getThread());
  }
}


function processTxnEmails() {
  var messages = getBankEmails();
  var txnData = parseMessageBody(messages);
  saveToSheet(txnData);
  labelMessagesAsDone(messages); // add this
  return true;              // and this.
}


// FILTERING EMAILS...

function getBankEmails() {
  // This function searches for messages from emails having subject containing
  // “GeNS transaction Alert … and your chosen label”

  var filter = "subject:GeNS transaction Alert AND -label:scripting-emails"; // add label 
  var threads = GmailApp.search(filter, 0, 10);
  var messages = [];

  for (var x in threads) {
    var thread = threads[x]
    messages.push(thread.getMessages()[0]);
  }
  return messages;
}