This is a nice way of having mailto: links open in Gmail with Debian and GNOME (it’ll probably work with other online email providers and other Linux distributions and other desktop environments).

All you need to do is create a file named gmail.desktop in the /usr/share/applications directory (you should probably be able to install it locally in ~/.local/share/applications, if you prefer) and add the following:

Then ‘Gmail’ should be listed as an option in Gnome Settings > Details > Default Applications (you may need to run sudo xdg-desktop-menu forceupdate first).

Note, here I’m using Debian’s sensible-browser command to launch the default web browser.