Your great uncle is the brother of your grandmother or grandfather. Therefore you and your great uncle's grandson share one or two great grandparents, and you are both the same number of generations separated from that shared ancestor.
So you are second cousins to each other.
Your great grandfather's brother is your great uncle and your grandchild's great-great-great uncle
Your grandfather's brother's grandson is your second cousin.
The children of siblings are first cousins.
The grandchildren of siblings are second cousins.
You would be third cousins. You share a common Great Great Grandparent.
second cousin/once removed
He's your great-great-grandfather.
Second cousins.
You would be third cousins.
He would be your great-great uncle.
Your grandson is the great-nephew of your brother.
Your grandfather's brothers would be the uncles of your parent. That would make them your great uncle.
You Aunt is you grandson's Great-Great Aunt. Your grandson is your Aunt's Great-Great Nephew.
Your brother is a great great uncle to your great grandson. Your great grandson is your brother's great great nephew.
What is your great-great-great grand nephew?!
Your sister is your grandson's great aunt. He is her great nephew.
Your great aunt's grandson is your second cousin.
She would be your great grandmother
Your niece's grandson is are your great-grandnephew.
Your sister's great grandson is your great great nephew. You are his great great uncle, or great great aunt.
Your great grandmother's brother's great grandson is your third cousin.