The Digital Inspiration weblog has a step-by-step for adding HTML to Gmail messages.
For those of you who have tried inserting an image or table in Gmail, you’ve probably noticed that Gmail won’t really allow embedded HTML (at least not in a traditional sense). In order to embed HTML, you actually need to copy the rendered HTML from either an existing web page or WYSIWYG HTML editor and then paste it into Gmail. Doing so allows you to insert your own tables, divs, images, etc. into Gmail pretty easily.
Personally, I’m not a huge fan of embedded anything in emails (give me plain text, please!), but for those of you who have been missing HTML in Gmail, Digital Inspiration’s method should do the trick. If that method isn’t your cup of tea, you might also want to try the Gmail Skins Firefox extension, which allows you to embed HTML directly in Gmail.