You are your great nephew's great uncle or great aunt, depending on your sex.
You're a Great Aunt to your nephew's son, who is your great nephew.
You are your nephew's uncle, or aunt, and his son's great uncle, or great aunt. Your nephew's son is your great nephew.
Your nephew's grand son is your great great nephew and you are his great great uncle (or aunt)
Your grandfather's great nephew is your second cousin.
The English language has no relationship name for the wife of your great niece or great nephew.
Your grandfather's great nephew is your second cousin.
In English language terms for family relationships, there is no relationship called "nephew once removed." The son of your nephew is your great nephew.
Your nephew's daughter is your great niece and you are her great aunt (if you are a woman) or her great uncle (if you are a man).
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.
Great nephew
Your great nephew's children are your great great nieces and great great nephews. You are their great great aunt or great great uncle.
There is no difference between a great nephew and a grand nephew. In some places "great nephew" is the preferred term for the grandson of one of your siblings. In other palces, grand nephew" is the preferred term for that relationship.