回复：Could you find any USCIS documents stating this information?

I do not find the passport validity duration for green card holder, but find the rquirement for non-immigration visa holder in CBP website and it is called "Passport requirement to be valid for six months beyond the period of intended stay in the US".

From this definition, you can easily tell it does not applys to green card holder because a GC holder can stay in US permanently, not has a specific "the period of intended stay in the US".