Someone I know has apparently done some research on this. She says that while there have been societies that have accepted gay people and where it has been very public, the concept of gay marriage is new (at least in societies that had the concept of marriage as we know it).
Is she right about that?
You're going to have a rough time finding any discourse on the phrase "gay marriage" that doesn't violate the 20 year rule. Among other things, even relationships that would have been considered "gay marriages", like Boston marriages, they would NEVER have been married in a Christian sense.
Basically, you're on the right path, but framed as such you're not likely to get any interesting answers. I would recommend asking about the origin of the modern gay marriage movement: that is more likely to bear fruitful answers.