slang from Cork city in Ireland - short for "go on". The term made it's made to the Carribean while Irish slaves were shipped there in the 16th and 17th centuries. Slang Irish terms like this then made their way into the Carribean "patois", particularly Jamaica, where "gwan" is pronounced and used in exactly the same way as in Cork city in Ireland.
"Ah gwan ya langball ya"
"Please do go on about that wonderful event that you are informing me about."