The French word for 'duke' would be 'duc' (pronounced more or less identically to the English word, for the obvious reason that the English word is borrowed from French). The German word appears to be Herzog. It's hard to see where either word might be plausibly confused with any possible pronunciation of "grave".

https://fr.wikipedia.org/wiki/Grave_(titre) somewhat equivalent of count or earl

You are right that it wasn't the equivalent of duke. I was being a bit imprecise. Sorry.

The better English equivalent to German Graf would have been an earl.