Your grandfather's niece is your parent's first cousin. Her daughter is your second cousins and her granddaughter is your second cousin, once removed.
You would be second cousins.
Your father's nephews and nieces are your first cousins.
Grandaughter.
Your grandfather's niece is a first cousin of one of your parents. Her son is your second cousin.
Your grandfather's brothers would be the uncles of your parent. That would make them your great uncle.
step-grandaughter
She will be your aunty
between your grandaughter husband, and you? if you have a granddaughter husband, you may not want to make that public. i don't know if that's legal.
Your granddaughters and son have the relationship of nieces and uncle. Your granddaughters being his nieces and your son being their uncle.
Your daughters are great aunties if their nieces have kids. Your daughters kids are your grand children.
He is their Uncle and they are nieces and nephews.
Two people whose grandfathers were brothers are second cousins to each other.