You can set an email to send in plain text mode by clicking the "3 vertical dots" button to the right of the discard/trash button in Gmail's compose window.
Note: the "down arrow" button circled in the screenshot below has been changed to 3 vertically positioned dots
In the menu that appears as seen above, click Plain text mode. If you click the "3 vertical dots" button again, you'll see the check next to "Plain text mode" which lets you know you are composing your email in plain text.
To revert back to the rich text format, go back into the menu and click on Plain text mode. If there is NO check next to "Plain text mode", you are composing your email in rich text format.