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 grandfather's great nephew is your second cousin.
What is your great-great-great grand nephew?!
Your great great grandfather.
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 great nephew is your kids' first cousin once removed.
Your grandson and your great nephew are second cousins to each other.
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.