Judging by the events, you should also be able to open the save file and check the global flags directly. It should be a bit faster than rushing the insights to identify the preset outcome. Poking at the events, it appears to be governed by the following four global flags:
50 weight: Nanite Crisis (set global flags "gray_goo_crisis_set" and "active_gray_goo")
30 weight: Dragon Leviathan (set global flag "dragon_season")
30 weight: Nanite Empire (set global flag "gray_goo_empire_set")
30 weight: Abandoned Cluster (set no relevant global flags)
Opening a savefile is a matter of going to the save directory, opening the save file in something like 7zip or WinRAR, and opening the gamestate file in a text editor. That said, I've never tried to force a particular L-Cluster outcome, so I've never tried verifying it. Most games, I don't even end up with an L-Gate in my territory.
EDIT:
Also, since we're talking Ironman, I'd think you can open it without altering anything the game can check, but editing the global flag would likely be a bit more problematic. I don't play Ironman either, so I can't be of particular help on that front either. My apologies.
EDIT EDIT:
Also also, worth noting though it's likely obvious: the L-Gate cluster and contents are spawned when you crack open your first L-Gate, if I'm reading the events right (distar.11000, triggered by distar.10950, triggered by the project LCLUSTER_PROJECT). After the first empire finishes the project to open the L-Gates, it's too late to change it.