Your niece and your grandson are First cousins, Once Removed.
great great nephew
and u wud be a great great aunt... i think :)
Your grandfather's niece would be your parent's first cousin. They would be your first cousin once removed. That is because you are one generation away from each other.
your the great great aunt or uncle
Grandniece
Great grand-nephew
Your niece's grandson is are your great-grandnephew.
Your niece's child and your grandson's child (your great grandchild) are second cousins, once removed, to each other.
Your grand niece and your grandson are second cousins to each other
Your niece and one of your grandson's parents are first cousins. That makes your niece a first cousin once removed of your grandson.
A great niece and a grandson of the same person are second cousins to each other.
Your great grandson and your husband's great niece are second cousins, once removed.
Your niece's child and your grandson's child (your great grandchild) are second cousins, once removed, to each other.
Your niece's son is your great-nephew. Your niece's grandson is your great-great nephew.
You are his great-great-aunt (or great-great-uncle) and he is your great-great-nephew.* A much less-used US term for the relationship is great-grandnephew which follows the logic that your niece's grandson is your sibling's great-grandson, so would have the same generation-separation from both you and your sibling.
second cousin
Your aunt's great grandson is your first cousin twice removed.
Your grandson and your great great niece are second cousins, once removed, to each other