Someone born before it was even legal to be gay, who has worked hard to get us the equality and legal recognition we enjoy today.
"Jim's a real gaytriarch. He claims to have been the first publicly "out" gay man in Michigan, and even though he's almost 90, he still works hard for the rights of 'TBLG' people."
A parent or grandparent, who used to be straight, or at least straight enough to have procreated (or adopted).
"I dropped mom off at the Out Bar. It's gaytriarch night."