When I try to publish content from Google Spreadsheet, it puts it into an HTML iframe. This is found under
File > Publish to Web.
How can I send this iframe content in a mail, sent over Gmail?
Writing HTML in the Gmail is not possible so you'll have to use another client to compose your email. But especially for
iframe I don't think that anyone will be able to see it, since it has some security issues and most likely Gmail will block the contents of it. Consider sending it as an image.