Gmail Touch+ for Windows 8 is a third party email client that allows you to interact with your Gmail account from Windows 8's Metro interface.
Gmail Touch+ for Windows 8 also supports Live Tiles, notifications, and multiple Gmail accounts. It also includes a shortcut to the desktop view to use its advanced features.
Since Gmail doesn't have a Windows 8 app yet, Gmail Touch+ for Windows 8 is your best bet for using Gmail from the Metro interface.
Download Gmail Touch+ for Windows 8 in Softonic