You should definitely use both plain text and HTML. What GMail might do is of little consequence, what matters is what ALL email readers will do, and since you can't
test that you need to be on the safe side and send a plain text version as well. Furthermore, some people will turn off HTML in emails because they don't like it.
In order to send both plain text and HTML in the same mail you need to use
attachments.
This tutorial will get you started with that.