Here we’ve saved the best piece of speculation for last. In Sworn to the Sword, Pearl mentions that the first battles for Earth took place in the sky arena. Rose’s Scabbard makes it rather unlikely that Pearl could be referring to the rebellion, but there’s another possibility: it wasn’t the rebellion that started in the arena, but a contest for the right to colonize the Earth. The most likely opponents for the Gems to face in these opening battles would be humans.
What we know
The sky arena was clearly built before the rebellion: it shows Rose Quartz as being a ruler of Gemkind, which clearly means it was built before she broke away. We also know that whatever the first battles for Earth were, they were before Pearl learned to fight and either before she followed Rose Quartz or before she dedicated herself to Rose’s service so completely, as Pearl learned both to fight and to zealously serve while the battles were taking place in the arena.
By the time of the rebellion Pearl was already dedicated to Rose Quartz and was already a warrior. There was likely no legal cover for their rebellion, as Rose warned Pearl that siding with her meant either death in defeat or exile in victory—not terms one would expect if it were the polite kind of conflict that took place in the arena.
The arrival of Gemkind
We know that the Kindergarten was active six thousand years before the present day in the series. Assuming that their timeline is roughly like our own and the show’s present day is around the same as ours, as we previously established that would put the activation of the Kindergarten before the Bronze Age had started.
Though the great civilizations would not have risen yet, there would still be rulers with professional warriors at their command who would take notice of the arrival of Gemkind on Earth. The Gems could easily swat any of their efforts aside, but they are clearly a martial civilization with no small amount of pride; perhaps making a contest of it appealed to the Gems.
The sky arena could have been built far above the ground specifically so they did not take any land claimed by humanity “unfairly”. From there they could issue their challenges to humanity, pitting Earth’s greatest warriors against whatever champions the Gems chose to put up. Maybe they only sent warrior Gems against the humans, and Pearl joined the cult of the warrior just by observation. Perhaps the Gems were boastful, and Pearl or non-combatant Gems like her were also sent in to show that any Gem could beat any human.
The outcome of the battles is clear: the Gems would eventually build structures all over the planet. They didn’t want the whole planet, at least at first, and their infrastructure appears to be quite light; dozens of warp pads link the Gem sites together, but the actual percentage of Earth controlled by Gems was small. This would make humans less likely to bother with the Gems: their warriors are too strong to defeat, so if they are taking so little land then it’s probably best just to leave them alone.
She made me a knight
The one aside confusing the chronology is Pearl labeling the warrior tradition she learned as being specifically that of knighthood. This is, as most readers will be aware, a specific warrior tradition that was limited to medieval Europe thousands of years separated from the Bronze Age.
This doesn’t really matter much, though. First of all, the chronology is quite clear: Pearl learned about “knighthood” before the rebellion, which happened more than five thousand years ago.
Secondly, as Pearl told Steven in Mirror Gem, there are many ways to say the same thing. Most readers are probably also aware of the samurai, a distinct warrior tradition that was broadly similar to the knights and very geographically separated from them. Just because Pearl came to associate the ideals she learned with the term “knight” doesn’t necessarily mean that the term existed when she learned how to be a warrior. Professional warriors have served powerful individuals for a very long time—Pearl isn’t a human and doesn’t follow human culture very closely, so she’d be very prone to conflating these sorts of things.
There’s even the slight possibility that the history of Steven Universe’s Earth is very different from ours. The Armor of the Fallen in Rose’s armory is made in a variety of very human styles (including suits of armor that would be worn by knights and samurai), but these styles postdate the rebellion by thousands of years. Rose could have had any number of needs for human warriors who she would have reason to venerate after the rebellion (corrupted Gem monsters would probably have been more numerous in the past), making it likely that they are in fact much newer, but if the armor was in use during the first battles for Earth then they are highly anachronistic for our timeline.
I forget the other one
Of course it’s entirely possible that a key assumption of ours is wrong: the show’s present day may not be our present day. Knighthood may be less than two thousand years old to us, and those armor suits may only be six or seven hundred years old to us, but if humans participated heavily in the rebellion then more humans may have died than Greg implied.
There is a deep-seated apathy to Gems and magic in most humans. If the homeworld hit humanity hard enough to set back the course of civilization thousands of years, the only people that survived may have been the ones that didn’t get involved. Gems and magic may be a part of history to them, but it’s history that happened to other people. The Gems are here, but they’re not going to get too worked up about it.