Your great aunt's grandson is your second cousin.
Your grandfather's brother's grandson is your great uncle's grandson. The grandchildren of great aunts and great uncles are your second cousins.
it's like your 3rd cousin
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.
Your sister is your grandson's great aunt. He is her great nephew.
Your niece's grandson is are your great-grandnephew.
First cousins, twice removed. Aunts daughter is your cousin. Aunts daughters daughter is cousin once removed Aunts daughters daughters daughter is cousin twice removed.
Your great grandmother's brother's great grandson is your third cousin.
Your great aunt's great grandson is your second cousin, once removed.
Your sister would be your grandson's great aunt.
Your grandson is your brother-in-law's great nephew, and your brother-in-law is your grandson's great uncle.
Your brother's grandson is your great nephew and you are his great uncle if you are a man, or great aunt if you are a woman.