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.

PowerPoint 2007 saves presentations as Web pages, converting files to HTML with a single step. PowerPoint 2010 disabled this function, but it saves presentations as images which you can embed ...

Common image files include raster graphics such as Graphics Interchange Format (GIF), JPEG File Interchange Format, Portable Network Graphics (PNG), Tag Image File Format (TIFF) and Windows Bitmap.