Class GmailThread

// Add label MyLabel to the first thread in the inbox
var label = GmailApp.getUserLabelByName("MyLabel");
var firstThread = GmailApp.getInboxThreads(0,1)[0];
firstThread.addLabel(label);

// Create a draft reply to the message author with an acknowledgement.
var firstThread = GmailApp.getInboxThreads(0,1)[0];
firstThread.createDraftReply("Got your message");

// Create a draft response with an HTML text body.
var firstThread = GmailApp.getInboxThreads(0,1)[0];
firstThread.createDraftReply("incapable of HTML", {
  htmlBody: "<b>some HTML body text</b>",
  cc: "another@example.com"
});

// Create a draft reply to all recipients (except those bcc'd) of the last email in this
// thread.
var firstThread = GmailApp.getInboxThreads(0,1)[0];
var message = firstThread.getMessages()[0];
message.createDraftReplyAll("Got your message");

// Create a draft reply, using an HTML text body, to all recipients (except those bcc'd) of
// the last email of in this thread.
var firstThread = GmailApp.getInboxThreads(0,1)[0];
firstThread.createDraftReplyAll("incapable of HTML", {
  htmlBody: "<b>some HTML body text</b>",
  cc: "another@example.com"
});

// Log the subject of the first message in the first thread in the inbox
var firstThread = GmailApp.getInboxThreads(0,1)[0];
Logger.log(firstThread.getFirstMessageSubject());

// Log the subject of the first message in the first thread in the inbox.
var firstThread = GmailApp.getInboxThreads(0,1)[0];
var id = firstThread.getId();
// Get same thread by its ID.
var thread = GmailApp.getThreadById(id);
Logger.log(thread.getFirstMessageSubject() == firstThread.getFirstMessageSubject()); // True

// Log the names of the labels attached to the first thread in the inbox
var firstThread = GmailApp.getInboxThreads(0,1)[0];
var labels = firstThread.getLabels();
for (var i = 0; i < labels.length; i++) {
  Logger.log(labels[i].getName());
}

// Log the date of the most recent message on the first thread in the inbox
var firstThread = GmailApp.getInboxThreads(0,1)[0];
Logger.log(firstThread.getLastMessageDate());

// Log the number of messages in the thread
var firstThread = GmailApp.getInboxThreads(0,1)[0];
Logger.log(firstThread.getMessageCount());

// Log the subjects of the messages in the thread
var firstThread = GmailApp.getInboxThreads(0,1)[0];
var messages = firstThread.getMessages();
for (var i = 0; i < messages.length; i++) {
  Logger.log(messages[i].getSubject());
}

// Logs the permalink for the first thread in the inbox
var thread = GmailApp.getInboxThreads(0,1)[0];
Logger.log(thread.getPermalink());

// Log if this thread has starred messages
var firstThread = GmailApp.getInboxThreads(0,1)[0];
Logger.log('has starred : ' + firstThread.hasStarredMessages());

// Log if this thread is marked as important
var firstThread = GmailApp.getInboxThreads(0,1)[0];
Logger.log('Important? : ' + firstThread.isImportant());

// Log if this thread is a chat
var firstThread = GmailApp.getInboxThreads(0,1)[0];
Logger.log('is in chats? : ' + firstThread.isInChats());

// Log if this thread is in the inbox
var firstThread = GmailApp.getInboxThreads(0,1)[0];
Logger.log('is in the inbox? : ' + firstThread.isInInbox());

// Log if this thread is in the priority inbox
var firstThread = GmailApp.getPriorityInboxThreads(0,1)[0];
Logger.log("is in priority inbox? " + firstThread.isInPriorityInbox());

// Log if this thread is in the spam folder
var firstThread = GmailApp.getInboxThreads(0,1)[0];
Logger.log('Spam? ' + firstThread.isInSpam());

// Log if this thread is in the trash
var firstThread = GmailApp.getInboxThreads(0,1)[0];
Logger.log('Trashed? ' + firstThread.isInTrash());

// Log if this thread is unread
var firstThread = GmailApp.getInboxThreads(0,1)[0];
Logger.log('Unread? ' + firstThread.isUnread());

// Mark first inbox thread as important
var firstThread = GmailApp.getInboxThreads(0,1)[0];
firstThread.markImportant();

// Mark first inbox thread as read
var firstThread = GmailApp.getInboxThreads(0,1)[0];
firstThread.markRead();

// Mark first inbox thread as unimportant
var firstThread = GmailApp.getInboxThreads(0,1)[0];
firstThread.markUnimportant();

// Mark first inbox thread as unread
var firstThread = GmailApp.getInboxThreads(0,1)[0];
firstThread.markUnread();

// Archive first inbox thread
var firstThread = GmailApp.getInboxThreads(0,1)[0];
firstThread.moveToArchive();

// Move first non-inbox thread to inbox
var firstThread = GmailApp.search("-in:inbox")[0];
firstThread.moveToInbox();

// Move first inbox thread to spam
var firstThread = GmailApp.getInboxThreads(0,1)[0];
firstThread.moveToSpam();

// Move first inbox thread to trash
var firstThread = GmailApp.getInboxThreads(0,1)[0];
firstThread.moveToTrash();

var firstThread = GmailApp.getInboxThreads(0,1)[0];
// ...Do something that may take a while here....
firstThread.refresh(); // Make sure it's up-to-date
// ...Do more stuff with firstThread ...

var myLabel = GmailApp.getUserLabelByName('<your label>');
var threads = myLabel.getThreads();
for (var x in threads) {
  var thread = threads[x];
  thread.removeLabel(myLabel);
}

// Respond to author of last email in thread with acknowledgment
var firstThread = GmailApp.getInboxThreads(0,1)[0];
firstThread.reply("Got your message");

// Respond with HTML body text.
var firstThread = GmailApp.getInboxThreads(0,1)[0];
firstThread.reply("incapable of HTML", {
  htmlBody: "some HTML body text",
  noReply: true
});

// Respond to all with acknowledgment to the first thread in the inbox
var firstThread = GmailApp.getInboxThreads(0,1)[0];
firstThread.replyAll("Got your message");

// Respond with HTML body text.
var firstThread = GmailApp.getInboxThreads(0,1)[0];
firstThread.replyAll("incapable of HTML", {
  htmlBody: "some HTML body text",
  noReply: true
});