Install HTML editor in Gmail from Chrome Web Store.
How to get started and troubleshoot this extension is explained in support note.
HTML Editor for Gmail
How to send HTML email in Gmail?
Since Gmail was introduced way back in 2004, many changes have been made to the email provider. One of the first changes was to remove the HTML editor and just keep the WYSIWYG editor. While Gmail doesn’t offer the functionality of an HTML editor, it does still support HTML.
Can I embed my iframe into an email?
Unfortunately, iframes aren’t currently supported by most email providers.