Australian Slang, (noun) A person that is totally lacking in fashion sense, motor skills and/or social skills. Usually a total moron, an extremely unpleasant person or an unwanted guest.
Sometimes: A likeable idiot or Bogan (especially in Queensland)
1. No wonder he can't get a girlfriend, he is such a gronk!
2. Why would you "steal my car for the weekend," you gronk!