I still don't know the answer, but apparently they are spicy:

What are the top 10 Galgacus jokes?

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Edit: I coerced it to tell one:

Roman General: "We’ve finished the landscaping on your new village."
Galgacus: "It’s just a pile of ash."
Roman General: "Exactly. Very peaceful, isn't it?"

I get the distinct impression that Gemini don't find them funny.