Can a great uncle be refereed to as a paternal ancestor

[DavidG02]

They are my fathers ancestors = paternal

They are on my mothers side = maternal

Maternal uncle , paternal gguncle. It defines the side of the tree

[Rosinish]

Does it make grammatical sense to say that a great great great uncle (on someones fatherline) is a "paternal ancestor"?

I would agree on this as others do as (in my opnion) they're still on a branch of your 'direct' ancestor.

What I don't agree with is when someone refers to their grand parents siblings as being a 'great aunt/uncle'...

A 'great' (to me) would be the sibling of my great grandparent i.e. if people refer to their grandparents' sibling then (to me) they would be my grand aunt/uncle but...each to their own although I find it far easier with how I refer to my ancestors siblings to keep with the prefix e.g....

My grandmother was Mary & my granduncle (Mary's sibling) was John

Annie