Amilius designed and created the SGU gates first in the milky way, but then they found out about that background microwave radiation and decided to create a path of automated seed ships and the Destiny to investigate and create a path to gather the information of this background radiation.
Then many generations later they eventually created the upgraded silverish gray version gates, likely using an improved seeding processas as their tech upgraded significantly at that point.
Amilius would have likely long since passed when this upgrade took place, but it could have been his great grandkids 100000x years later down the road to create this version, same with the Atlantis style gates as this information is all based on the 50 million year history of Alteran culture.
Going back to when the Ori created their version of Stargates, it's tough to say as we don't know for sure when they could have created them.
A theory is they could have obtained this knowledge when they spoke to Daniel Jackson and vala when he and her were using the communication stones.
Vala would have likely had this information buried in her brain as she was a host to the goa,uld quteche and she likely understood the technical elements of the Stargate, and the Ori likely learned through Daniel about his buried knowledge of the ascended Alteran as the comment to Daniel about them was that they shielded them and their existance from the Ori, then Daniel muttered to himself saying I didn't think they did much of anything.
And from then on out, was when we could see a prior stepping through the Milky Way star Gates.
I don't necessarily understand how they could have come up with a coordinates to dial these Gates, but the ascended Ori could possibly extract information like how a goa,uld does with genetic memory except through ascended knowledge gaining access to everything Daniel in Vala's lives.
They likely only created one Stargate when this information was discovered, and then dialed one of the gates that either Daniel or Vala could have had in there mind's.