As far as I can make out, Your daughter is your fathers (common ancestor) grandchild, and your grand nephew (not great nephew) is your fathers great grandson.
According to the chart at http://freepages.genealogy.rootsweb.ancestry.com/~gentutor/chart.html
this would make your grand nephew, your daughters First Cousin once removed
Your nephew's grand son is your great great nephew and you are his great great uncle (or aunt)
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.
You're a Great Aunt to your nephew's son, who is your great nephew.
Great uncle / Great nephew
The relationship sequence:Your nephew/niece is a child of your sibling and your son's first cousin.Your nephew/niece's son is your great nephew (or grand nephew) and your son's first cousin once removed.The grandchild of your nephew/niece (your great nephew's son) is your great great nephew and your son's first cousin twice removed.
The relationship sequence:Your nephew/niece is a child of your sibling and your son's first cousin.Your nephew/niece's son is your great nephew (or grand nephew) and your son's first cousin once removed.The grandchild of your nephew/niece (your great nephew's son) is your great great nephew and your son's first cousin twice removed.
Your niece's son is your great nephew. You are his great aunt or great uncle.
Great nephew
In English language terms for family relationships, there is no relationship called "nephew once removed." The son of your nephew is your great nephew.
it great nephew :)
Your nephew's son is your great nephew. Your niece's son is also your great nephew.
The son of your great niece is your great great nephew. You are his great great uncle.