Justinian had a runaway victory, but there are many reasons to believe it would be hard to reproduce. Settling west, he took a big bite out of Catherine's land, but settling east would have breathed life into her game. Even late in the early game, Justinian was slow to metals, and had not expanded very well. We can see from the turn-priority silk settle that Justinian could be very squeezed in other playthroughs. While the monopoly on culture and wonders made sense in this game, religious hegemony seemed somewhat of a high roll.
I can definitely see this is not a 90% Gilgamesh map

He had a lot to do in the early game: settle the land, clear barbarians, and carry out early conquest. While he succeeded in the first two, he completely failed to gain ground, even losing a border city that would prove bitterly regretful during Bismarck's rampage! Personally, I was surprised to see Gilgamesh struggle to meet Genghis' unit production. It was also odd that chariots formed such a large proportion of his defenses when he had access to copper. The forever war eliminated any early advantage from his land, especially with his odd lack of production. He did have land, but it was indeed average. The tundra conquest seemed potentially limited by pathfinding.
Bismarck proved himself capable but not quite competent. He settled towards the jungle, expanded well, researched the Wheel, kept the economy afloat, and even became a bit of a monster! If anything, the game showed that Bismarck's start with adequate expansion could compete directly with Gilgamesh's land. It's easy to belittle Bismarck given his fatal final war declaration, but
he was the only AI to contest Justinian's early game(!) While I think his position would be weaker in the Alternate Histories, as Tokugawa proved an unexpected shield, his peace weight gave him options for mid-game expansion, and I do think he would advance a fair number of times.
As has been noted, Catherine expanded well, took land off of Genghis, and that was about it! Justinian's holy city made it difficult to take land away from him, though she tried. It seems clear this was a non-starter for Catherine--her AI just does not wait to plot war until she is down to one city! It's possible she needs a weak start from Justinian just to be relevant. While I think Catherine is a good AI, I agree she's lucky to have another chance.
I am probably most impressed with how Wang Kon held his own in the early game. He managed to split capital duties with his first settle, prioritized Hunting and Iron Working, and proved a tech leader with relevant stats into the mid-game. Unfortunately, his lack of early expansion, especially relative to Bismarck, meant that his land just could not compete against the larger empires. Given Gilgamesh's weaker start, it seems less likely Wang Kon would make much headway to the west.
It was also interesting that the area around Gilgamesh's and Wang Kon's first settles was fiercely contested. I wonder if it was to the defender's advantage to be able to concentrate their forces against the rotating sieges.
Tokugawa had an underwhelming performance featuring bizarre misuse of river commerce, skewed research priorities, and late access to metals. While he settled predictably away from the map, he struggled to expand to the point that Justinian added late filler cities. But as
Keler notes, we did not see much of Tokugawa in the stream anyway. I am inclined to view this as an anomalous result.
And then we have Genghis Khan who probably performed as some expected. While he lost almost the whole region north of his capital to Catherine, he
did settle into the map. Additionally, he took a city off Gilgamesh, cleaned up Tokugawa, and almost did the same to Catherine. I am not particularly surprised he folded so quickly to Bismarck. With a better start, I can see how his sheer unit production would overwhelm Catherine or Justinian.