Your niece and one of your grandson's parents are first cousins. That makes your niece a first cousin once removed of your grandson.
If you really need a relationship term, use granddaughter-in-law. Most people would merely say "my grandson's wife."
The grandson of your nephew is your great great nephew, but few people use that relationship as a form of address. Usually they call relatives by name or business title, not by relationship.
Your great-grandmother's sister's grandson is your second cousin once removed.
Rahul Gandhi is the grandson of Indira Gandhi.
second cousin
Your aunt's great grandson is your first cousin twice removed.
Your sisters grandson would be your grandnephew.
Your son and your aunt's child are first cousins, so your son and your aunts grandson (the son of your aunt's child) are first cousins, once removed.
it's like your 3rd 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.
Your brother is your grandson's great uncle, and your grandson is your borther's great nephew. Some people would also say grand nephew and grand uncle.
Your niece's child and your grandson's child (your great grandchild) are second cousins, once removed, to each other.
Your grandson and your brother's grandson are second cousins to each other.
Your niece's grandson is are your great-grandnephew.
They are your Uncles and or Aunts.
First cousins, twice removed. Aunts daughter is your cousin. Aunts daughters daughter is cousin once removed Aunts daughters daughters daughter is cousin twice removed.