function markGmailAsRead() {
  // Find Gmail Messages that are unread but not in inbox
  var threads = GmailApp.search('is:unread -in:inbox', 0, 50);
  for (var i = 0; i < threads.length; i++) {
    // Does the message thread have any user-assigned labels
    var labels = threads[i].getLabels();
    if (labels.length == 0) {
      // Mark the entire thread as read
      threads[i].markRead();
    }
  }
}

// Create a trigger that execute every hour and processes 50 messages in a batch
function createTrigger() {
  ScriptApp.newTrigger('markGmailAsRead').timeBased().everyHours(1).create();
}