
I mean I know the answer, but for those that don't want to see this spoiler just don't look!
I'm not sure when we'll learn this information, but Jupiter is Gaia's [the system] son; I think with the "father" guy. He was experimented on and Gaia had lost him, so when she integrated with the System, she made Gigyu into the vessel to allow her son to be reborn. If you momentarily forgot about Gaia, she was the one in the yellowish orb you see in the 3rd test on the 50th floor.

No, it's like a parallel world theory that happened when Gaia became the system. The Cronus of the timeline we saw appear was from the world he existed in, while the one Gigyu is in was a normal human, unlike the one with divinity. Granted, we're all still unsure what happened to Gigyu's father, but we know they look similar.
The two won't merge, the spoilers I read is that they become two separate entities with their own body.

Yeah, they're two different souls entirely, I think Gigyu was supposed to be just a fake persona inlaid for the vessel to grow until Jupiter could come to possess it, but something went wonky wrong and that didn't happen I think. I guess we'll know as the old lady tells us the story in the next coming chapters if 186 meant anything near the end. Granted, most of this is going off theory and with the little bit of spoilers that were given around this part. Most of the spoilers were about what we'd already read or bits and pieces here and there, as well as the couple outcomes in the end.
I've been following this webtoon with every update, but for the life of me I can't remember who he is or why he's in Gigyu's body *shoulder shrug*