Also, gmail is probably not going to let you send mail from an address other than your own. Note the error says:
all retries failed sending email to firstname.lastname@example.org from email@example.com
The from address is firstname.lastname@example.org so it must mean you configured the contact form to use the entered address as the from address, while this is allowed it is not recommended and may not work for gmail. Also you would need to put your gmail address as the default from address in site settings.
Hope it helps,