Phew, so I just spent time looking this up ok, I am pretty certain this is correct… And Thranduil is just the father of Gimizh’s uncle by marriage. It’s a fairly distant relationship, really.
There’s no special name that I can find. It appears that most folks call their uncle-in-law’s dad/mum by their name, or a nickname.
‘Great-uncle’ /‘grand-uncle’ refers to the sibling of your grandparents, apparently.

(marvel at my MS Paint magic k)
So yeah, pretty obscure/distant sort of relationship.



