I don't post to this specific list, so there might be weird issues I'm not aware of but...
Gmail has the plain text option behind the 3dot menu in the bottom right corner. Apple Mail deserves to die a fiery death and I refuse to support it but "Format" -> "Make plain text" *may* send a plain text message, or not depending on the OS X lottery. You can force Apple Mail with "Preferences" -> "Composing" -> "Message Format" -> "Plain Text" to compose plain text messages. Or you can use an actual mail client.
Gmail has the plain text option behind the 3dot menu in the bottom right corner. Apple Mail deserves to die a fiery death and I refuse to support it but "Format" -> "Make plain text" *may* send a plain text message, or not depending on the OS X lottery. You can force Apple Mail with "Preferences" -> "Composing" -> "Message Format" -> "Plain Text" to compose plain text messages. Or you can use an actual mail client.