Short for "God Damn" and it's usually used when something looks sexy. The person that made this term was YourRAGE and his fans, when they saw a sexy/"curvy" woman, he said GYAT and the chat did that too.
This is describing a person with the biggest, most curved, round ass mankind can ever perceive. It can be a compliment or an insult. It can defy a man as a fatass if they have a so called "gyat"