Gnome Gmail is a tool to configure Gmail in your default browser as your mail client, so clicking on a mail link in a webpage or document, will bring you to gmail in stead of opening a standalone mail client. This integration usually works also from the desktop file manager, for sending a file by mail.
Despite the name, it works not only with Gnome, but in all applications that support the http://freedesktop.org standards.
See Gnome Gmail