There is no such thing as an "opposite" of a great uncle.
The female form of great uncle is great aunt.
The person to whom you are a great uncle is your great niece or great nephew, depending on their sex.
None of these are opposites.
The opposite gender of a great uncle is a great aunt. A great uncle is the brother of one of your grandparents, while a great aunt is the sister of one of your grandparents. Both terms refer to relatives in the same generational level within a family tree.
The opposite relationship of uncle would be Niece or Nephew.(The opposite gender is aunt.)
The opposite gender of uncle is aunt.
Sister-in-law describes a female. Brother-in-law describes a male.
You grandson is great great nephew to your uncle. You uncle is great great uncle to your grandson
You grandson is great great nephew to your uncle. You uncle is great great uncle to your grandson
Your mother's great great uncle is your great great great uncle
Your great uncle (by dictionary definition) would be your grandparent's brother. His great uncle would be his grandparent's sibling. So it would work out to be your great great great uncle (3rd great uncle).
Your great uncle is the brother of one of your grandparents. If you are a boy, you are your great uncle's great nephew. If you are a girl, you are your great uncle's great niece.
Your uncle is the brother of one of your parents. Your great uncle is the brother of one of your grandparents. Your great great uncle is the brother of one of your great grandparents.
Aunt
uncle