Your great grandfather's nephew was your grandfather's first cousin. You are two generations removed from your grandfather and his cousin. Therefore you and your great grandfather's nephew are first cousins, twice removed.
Your nephew's son is your great nephew.
Your nephew's daughter's son is your great-great nephew. Your nephew's daughter is your great niece.
The child of your nephew is your great nephew (if a son) or your great niece (if a daughter).
Your nephew's child is your great nephew (if a boy) or great niece (if a girl). You are his or her great aunt.
Your grandson and your great nephew are second cousins to each other.
Your great nephew is your kids' first cousin once removed.
Your great nephew and your granddaughter are second cousins to each other.
great nephew
Your child and your nephew are first cousins. Your child and your great nephew are first cousins, once removed.
Great Grand Nephew
Your nephew's daughter is your great niece and you are her great Aunt (or Great Uncle).Correction:Your nephew's daughter is your grand niece. You are her grand aunt or grand uncle.
you would be the child's great aunty or uncle Therefore, they would be your great niece or nephew.