The children of your grandfather's brother are your first cousins, once removed. You can also call them by their names.
Your grandfather's brother is your great uncle.
He would be your great great uncle, but you could call him whatever endearing name you both prefer. my great grandfathers brother or my great uncle
Your grandfathers brother to you is your Great Uncle!
Your grandfather's uncle is your great great uncle. Your grandfather's brother would be your great uncle
You would be second cousins.
Your brother's child's children are your grandnieces or grandnephews.
Your brother's children call you their aunt if you are female, or their uncle if you are male. Their are your nieces and nephews.
할아버지 (Hara buhjee)
Your brother's wife is your sister-in-law, but she is your children's aunt.
Your father's grandfather is your great grandfather.
Children of your siblings are nieces and nephews. Children of your spouse's siblings are still called nieces and nephews, but you are not related to them by blood.
The children born to your siblings are your nieces and nephews.