Gmail App - www.gmail.com

If you want to get your Gmail messages instantly via push notifications, use Gmail App. Gmail App is an easy way to get your messages instantly via push notifications, read and respond online & offline, find messages quickly to saves your time and keeps your messages safe.

With the Gmail app you get:

An organized inbox: Social and promotional messages are sorted into categories so you can read messages from friends and family first.
Less spam: Gmail blocks spam before it hits your inbox to keep your account safe and clutter free.
15GB of free storage: with these pace, you won’t need to delete messages to save space.
Multiple account support: Use both Gmail and non-Gmail addresses (Outlook.com, Yahoo Mail, or any other IMAP/POP email) right from the app.

Gmail App for iOS

Best Gmail App for iOS is Gmail - email from Google, published by Google. You can download on Apple store for free.
Download Gmail App for iOS

Gmail App for Android

Best Gmail App for Android is Gmail App, published by Google. You can download on Google Play Store for free.
Download Gmail App for Android

Gmail App for Mac

There are some good Gmail App for Mac published by third-party such as: MailTab for Gmail, it has 2 versions: free version and paid version (pro version). Another good Gmail app is Sparrow, which has been acquired by Google, Inc. You can download MailTab for Gmail free version here.
Download Gmail App for Mac

Gmail App for Windows

There is a good Gmail App for Windows: Gmail for Windows, published by Buster Collings Software. You can download fore free here:
Download Gmail for Windows

Gmail App for Chrome

There are some good Gmail App for Chrome such as: Gmail, offered by mail.google.com, and Gmail Offline, offered by https://mail.google.com/mail/mu. You can download for free here.
Download Gmail Offline for Chrome

Gmail App for Firefox

Best Gmail App for Firefox is: Gmail Notifier, with more than 100,000 users. You can download Gmail Notifier for free here.
Download Gmail Notifier for Firefox

Sign in Gmail App using App Passwords

An App password is a 16-digit passcode that gives an app or device permission to access your Google Account. If you use 2-Step-Verification and are seeing a “password incorrect” error when trying to access your Google Account, an App password may solve the problem. Most of the time, you’ll only have to enter an App password once per app or device, so don’t worry about memorizing it. Follow our tutorial about Sign in using App Passwords to sign in to your Gmail App