GYAT can have two meanings:
"Goddamn" - This is the most common meaning of GYAT. It is often used as an exclamation of surprise or disbelief. It can also be used as a euphemism for the word "goddamn."
"Get your act together" - This is a less common meaning of GYAT. It is used to tell someone to stop being lazy or disorganized and start doing what they are supposed to do.