Your Grandfather or your Great Uncle.
She would be your great grandmother
What is your great-great-great grand nephew?!
He would be your great-great uncle.
Your grandfather's great nephew is your second cousin.
My great great second, half, brother in law who's great great grandfathers....son.
Your great grandfather's sister is your great great aunt. You are her great great niece or great great nephew, depending on your sex.
Great uncle / Great nephew
Your nephew's grand son is your great great nephew and you are his great great uncle (or aunt)
She will be your aunty
your son is your great grandsons grandfather.
The son of your great niece is your great great nephew. You are his great great uncle.
He would be your son's third cousin.