Wmail

Note: WMail is no longer actively developed and has been superseded by Wavebox
Having to deal with multiple Gmail email accounts within a web browser can become somewhat counterproductive with all the tabs that usually start stacking up one after another.
A solution to this problem is offered by WMail, a streamlined piece of software that brings your Gmail and Google Inbox from your web browser directly onto your computer's desktop.
Subsequent to a typical and surprise-free installation, you are met by an empty main window where you are required to input your official Google credentials in order to log in.

User-friendly client that brings Gmail to your computer's desktop

As mentioned before, you have the option to choose from adding a Gmail and a Google Inbox instance, both requiring the same credentials. Please note that you can add any number of other instances for other Google accounts.
Getting to grips with this app is not challenging in the least, as it basically brings you the web-based interfaces of the previously mentioned Google services with which you are clearly already familiar.

User-friendly and surprisingly customizable Gmail and Google Inbox app

You can add and access all your Google accounts from the app's sidebar, found on the left side of the main window, which is also the only GUI element that really stands out on its own. Please note that from the same sidebar, you can also access the app's Settings section.
From this section, you can customize the way WMail behaves. For example, from the General tab, you can enable or disable the tray icon, activate unread number badge, enable the spell checker, turn the notifications sounds on or off, choose a default download location and hide GUI elements like the sidebar and the menubar.

Not impressive but definitely useful and worth your while

Taking everything into consideration, WMail is a practical application for everyone who wants to have quick access to Gmail or Google Inbox directly from their computer's desktop.
The application runs smooth, comes with a modern-looking interface and boasts enough simple and novice-accessible features to make it worthy of your attention, regardless your computer experience.