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).
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 great uncle
He would be his great great great uncle
Yes, he is their great uncle.
Your great uncle's grandchild is your second cousin.
Your grandfather's brother is your great uncle and your parent's uncle.
She is his great grandniece.
Your great great uncle's great great grandson is your third cousin once removed.
Your aunt's great uncle is also your parent's great uncle. So 2Great Uncle would be one way of naming them.
Great Uncle.
You are their great aunt/uncle.