Or (and this is just a personal theory) the Founder initially tried to search the Gems of Doom on his own after learning of them through Idriel or his own research, using the Exit Portals as markers for worlds he had already explored and the Praetorians as additional eyes to aid in the search. Then when he finally found his first gem (which is the only one he actually has and the one he's using as the prize for the Great Hunt), he probably discovered it in a world he already explored but initially thought was a dead end, which led him to the realization that it took him almost a lifetime to find just one when he still had 9 to go. At that point he prolly started to look for ways to speed up the process until he came up with the bright idea of outsourcing the hunt to millions of clueless normies, and thus the "VRMMO" and implant system were created.
Also during this time Caly, who was likely worldhopping on her own (either as a hobby or her way of training her portal powers or a way to temporarily escape her duties as a princess and just be her own person), likely came across one of the Exit Portals that the Founder had installed in one of the worlds. However she discovered that nothing happened upon touching it, and so she thought of it as nothing more than an odd structure before moving on to other worlds (until she eventually got captured).
That's definitely plausible, yeah.
I have been wondering about the gems of doom too. The one in Red Herring was implanted in a storyline and given by an NPC, which would suggest that someone with power over the NPCs placed it there, and the first thought of that would be the Founder, but why does he want everyone to find the gems if he'd have them already? So he probably didn't plant them, in my mind. The only thing I could think of that would explain that is that it really does serve as some sort of test to see if someone's "worthy" or powerful enough. I guess too, he doesn't just want ten gems of doom found, but he instead wants one single person to gather all ten of them. If he just wanted them found for him, presumably he could just nab them out of everyone's inventories once they're all found, or something to that effect (getting praetorians to gather them from the holders, for instance).
But if it's not the Founder, then maybe Idriel hid them all around? Maybe to keep them safe from the Founder? Don't know.
I do also maybe think that the Red Herring gem could have been planted by the Syndicate, in order for us to find it in that specific way, where we see both the beauty and the horror of the NPCs, which the Syndicate seems to want freed. But probably not, because the Founder did an announcement when we found that gem, but wouldn't have if it had been previously discovered by the Syndicate or whoever. I wonder too why the Syndicate wanted us specifically to find it, and why they wouldn't just do it themselves. I suppose they know something about the MC that we don't, maybe that would explain it.