Create the file $HOME/bin/mailto with the contents:

#!/bin/sh
gnome-open "https://mail.google.com/mail?extsrc=mailto&url=$*"

Make the file executable.

On Ubuntu Linux (using the Gnome desktop), go to:

System > Preferences > Preferred Applications

Under Internet / Mail Reader select “Custom” and enter the command:

/home/preston/bin/mailto %s

(Replace “/home/preston” with your $HOME.)

This should open GMail in your default web browser, composing a new message, with the recipient set.