You and your great aunt's grandson are second cousins. Your great aunt was the sibling of one of your grand parents, so your parent and your great aunt's child were first cousins. You and the grandson are members of the next generation and therefore second cousins.
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
The son of your grandson is your great grandson. You are his great grandmother or great grandfather.
Your grandmother's sister is your great aunt. (Some would say grand aunt). Your great aunts' grandchild is your second cousin. Her great grandchild is your second cousin, once removed. .
yes
You and your great great aunt's great grandson are third cousins.
You and your great uncle's grandson are second cousins.
great nephew
Your grandson and your great great niece are second cousins, once removed, to each other
Your daughter and your uncle's great great grandson are second cousins, three times removed.
Your great great great uncle was the brother of one of your great great grandparents. His grandson is the second cousin of your grandparent who is descended from that great great grandparent. Therefore his grandson is your second cousin, twice removed.
Your great-grandmother's first cousin's grandson is your third cousin once removed.