I don’t expect to get one definite answer here, but Google can’t give me one either. Some websites say they’re your second cousin, and others say they’re your first cousin once removed. I’m trying to figure out some family tree stuff so I need to know the difference between the (groups of) terms.
Great grandfather’s sister’s grandson is your second cousin once removed. That guy is the second cousin of one of your parents because they share great grandparents with one of your parents. A grandparent’s sibling is a great aunt or great uncle to you. A great grandparent’s sibling is a great great uncle or great great aunt to you.