Run Simplify / Gmail as a standalone app using Nativefier.
This is useful if you want to be able to keep your email separate from your million other browser tabs.
curl -o- https://raw.githubusercontent.com/davidguttman/simplify-gmail-native/master/install.sh | bash
Or just grab install.sh and run it.
This assumes you're running OSX, but you should be able to tweak the install script so that it works on Windows (replace the gmail.icns file with a PNG, and download the script.js and style.css files manually, etc...)