Uncles and aunts by marriage are still uncles and aunts, and people don't normally draw a distinction between "uncle by blood" and "uncle by marriage" - so the same goes for great-uncles and great-aunts.

The uncle of your grandfather would be a great-great uncle. Or alternatively a first cousin thrice removed.
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The female equivalent of "avuncular" is, I understand, "materteral" - but it's not a word that is exactly commonly in use.