Email clients that support Gmail aren't exactly rare, but the vast majority of them use the IMAP protocol to retrieve and manage your messages, which means they can't take advantage of Gmail-specific features such as inbox categorization, labels, and synchronized signatures.

Mimestream goes about it differently — it uses the Gmail API, allowing it to provide most, though not quite all, of the features you are familiar with from Google's mail client. It's also worth noting that the app is written in Swift, so it will integrate very well with macOS and provide a much smoother experience than, for instance, an Electron-based app.

Airmail 5 is a new mail client designed with performance and intuitive interaction in mind optimized for macOS High Sierra! Support for iCloud™, MS Exchange, Gmail™, Google™ Apps, IMAP, POP3, Yahoo!™, AOL™, Outlook.com™, Live.com™

Postbox 7 is a new email application that helps you organize your work life and get stuff done. It has all the elegance and simplicity of Apple Mail, but with more power and flexibility to manage even the most demanding workloads.

Kiwi for Gmail 2.0 integrates all of G Suite (formerly Google Apps) with Gmail as the centerpiece. We turn Gmail and the rest of G Suite into a full-powered desktop client that takes everything you know and love about Gmail and apps such as Docs, Sheets and Slides, and add great new features – including the ability to simplify your life by seamlessly managing multiple Gmail accounts. Finally a solution for office productivity that competes with Microsoft Office 365.

Postbox is a new email application that helps you organize your work life and get stuff done. It has all the elegance and simplicity of Apple Mail, but with more power and flexibility to manage even the most demanding workloads.

Unibox is a people-centric email client that organizes your messages by person. It avoids having email like a to-do list.

Mailplane brings Gmail to your Mac desktop. Mailplane combines Gmail's spam protection, endless storage, global access, and unique user interface with great Mac features

Boxy 2 is finally here. This is a minimal and powerful e-mail client you’ll fall in love with, and it’s been rebuilt from the ground up with new features and themes, a thoroughly refined interface and countless improvements.

CloudMagic works with Gmail, Exchange, Yahoo, Outlook, iCloud, Google Apps, Office 365 and any IMAP accounts. Sign up once and access your email across devices with ease.

Airmail 2 is a new mail client designed for Yosemite with fast performance and intuitive interaction.
Support for iCloud™, MS Exchange, Gmail™, Google™ Apps, IMAP, POP3, Yahoo!™, AOL™, Outlook.com™, Live.com™