What you use to set the mime type is not as expected (in addition, the content of the message should be in body, not head). Use something like the following:
mailMessage := MailMessage empty.
mailMessage setField: 'subject' toString: 'Trying to send html '.
mailMessage addMixedPart: '<html><head></head><body><i> Dear</i> </body></html>' contentType: ZnMimeType textHtml printString.
ZdcSecureSMTPClient sendUsingGMailAccount: 'you@gmail.com' password: 'XXXXXXX' to: 'recipient@server.com' message: mailMessage.
, text message.