Your aunt's grandson is the son of your First Cousin and is therefore your First Cousin once removed
Ref: http://freepages.genealogy.rootsweb.ancestry.com/~gentutor/chart.html
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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.
Kublai Khan was Gengis Khan's grandson