Hi,
Editing HTML source in the message compose is not available in Gmail.
However, you could able to use your favorite word processor such as MS
Word or creating an HTML file using your text editor such as notepad
then open it suing your browser then select all content then copy and
finally paste it into Gmail compose panel. It will work fine.
However, this is tested on Windows XP, IE 7. Also, background images
and graphics will not appear in the message, else you place them on a
public server the include them using their URLs.