You could easily be unrelated to your grandfather's cousin's second cousin.
If the two of you are descended from a common ancestor, the closest relationship possible between the two of you is second cousin, twice removed.
If your grandfather's cousin is not his first cousin, then the relationship is more distant.
Your grandmother is first cousins with cousin. Your parent would be the second cousin of the cousin's son. That would make you second cousins once removed.
Your second cousin's siblings are also your second cousins. The children of your second cousins are your second cousins, once removed.
second cousin
You and the child of your second cousin are second cousins, once removed.
Your second cousin and your son are second cousins, once removed.
Your second cousins.
Your children and your first cousin's children are second cousins. Your children and your second cousin's children are third cousins. And so forth.
Your grandmother is first cousins with cousin. Your parent would be the second cousin of the cousin's son. That would make you second cousins once removed.
Your first cousin's children are your first cousins, once removed. They are your children's second cousins. Your second cousin's children are your second cousins, once removed. They are your children's third cousins. And so forth.
Your grandfather's niece is a first cousin of one of your parents. Her son is your second cousin.
Your first cousin's child and your child are second cosins. Your first cousin's grandshild and your child are second cousins, once removed. Your second cousin's child and your child are third cousins. Your second cousin's grandchild and your child are third cousins, once removed.
Your cousin's sister is also your cousin. Your first cousin's children and you children are second cousins. Your second cousin's children and your children are third cousins. And so forth.