The grandchildren of siblings are second cousins to each other.
There is no relation between you and your grandmother's cousin-in-law's grandson.
Your father's great grandson could be your grandson, or your great nephew.
You and your uncle's grandson are first cousins once removed.
Your grandson is your sister's great nephew and she is his great aunt.
You grandson is great great nephew to your uncle. You uncle is great great uncle to your grandson
You grandson is great great nephew to your uncle. You uncle is great great uncle to your grandson
Your father and your aunt are siblings (brother and sister) The grandchildren of siblings are second cousins to each other.
Your grandson and your great nephew are second cousins to each other.
Great Grand Nephew
Great Uncle.
You are their great aunt/uncle.
Second cousin.