Linux (GNOME-based): There are workarounds to set Gmail as a default mail app in Linux, but they don't cover right-click file sending and complex mail links. Gnome Gmail does a much better job of integrating Gmail.
Download a DEB (Ubuntu/Debian) or RPM (Fedora/openSUSE) package from the site, install it, and then head to your Preferred Applications preference (in Preferences, under the System menu in Ubuntu, for example). Set Gmail as your "Mail Reader", and now it should really respond like a desktop client — on email links in every browser, when you right-click a file and select "Send to", and with full functionality when email links include cc: or pre-written subject lines. Google Apps users, you're not left out in the cold — hit the Configure link when you're asked to sign into Gmail on a right-click Send To action, and you can add your Google Apps login and make it the default.
Gnome Gmail is a free download for Linux systems running a GNOME desktop.
Gnome Gmail [SourceForge]