t'wara mein
Uncle is ewythr in Welsh.
Ewythr is 'uncle' in Welsh (Cymraeg).
Ewythr
Great > [Welsh] mawr
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.
It is "ewythr", but some Welsh speakers have some difficulty pronouncing it, so it is not uncommon to hear "wncl" being used colloquially, especially in South Wales where we tend to be less strict (or formally correct!) with the language.
Your grandfather's uncle is your great great uncle. Your grandfather's brother would be your great uncle