You can set an email to send in plain text mode by clicking the vertical ellipsis ⫶ button to the left of the discard/trash button in Gmail's compose window.
In the menu that appears as seen above, click Plain text mode. If you click the vertical ellipsis ⫶ 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 to the rich text format, go to 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.