To my knowledge, and after doing some web research, it is not possible to make the Gmail web interface do forced conversions to plain text when a message has been sent in HTML format. This actually makes sense since so much actual information is often contained in HTML elements like click-through links that are part of an image and similar.
You can choose to compose in plain text, but even that has become more difficult to set up since HTML formatted messages have become the de facto standard.
Brian - Windows 10 Home, 64-Bit, Version 1703, Build 15063 (dot level on request - it changes too often to keep in signature)
The opposite of a correct statement is a false statement. But the opposite of a profound truth may well be another profound truth.
~ Niels Bohr