Your niece and one of your grandson's parents are first cousins. That makes your niece a first cousin once removed of your grandson.
second cousins
Your grandniece is the granddaughter of your brother or sister, and the daughter of your niece or nephew. Your grandnephew is the grandson of your brother or sister, and the son of your niece or nephew.
The son or daughter of one of your siblings children is your great niece or great nephew. In other words, the grandson or granddaughter of your brother or sister is your great niece or great nephew.
If you really need a relationship term, use granddaughter-in-law. Most people would merely say "my grandson's wife."
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.
Your great-grandmother's sister's grandson is your second cousin once removed.
Your niece's grandson is are your great-grandnephew.
Your grand niece and your grandson are second cousins to each other
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 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.
Your grandson and your great great niece are second cousins, once removed, to each other
Your grandson and your sister's grandson are second cousins to each other.
Your niece is the daughter of one of you siblings. Your great grand niece is the great granddaughter of one of your siblings.If your niece and your great grand niece are both descended from the same one of your siblings, then they are grandmother and granddaughter to each other. If they are descended from different siblings, they are first cousins, twice removed, to each other.
Your wife's niece is also your niece, and her daughter is your great niece. It is to be hoped that you do not have a relationship WITH her, merely TO her.