It says that he was the last survivor of his people that were killed off by witch hunters.
And throughout history there have been many many big events that would have drawn humans to see them not dying from disease and their powers.
Gabriel is the only one known to be immortal, and he had been hiding for 600 years and his people died 6 to 700 years ago.
Their an offshoot or subspecies of mutants with a less evolved X-Gene, and Gabriel only survived cause he was the strongest among them.
Their numbers were only in the 300's and who knows how long it took for them to reach that number before being wiped out.
So as a subspecies they may have been around for 100 or 1000 years before they were killed off. And who knows how they know they were the same and why they came together.
Cause again we only know for sure Gabriel was immortal and that his people were. immune to the plague.
Was he born during this event, likely given he makes to mention of being around longer then that. His people didn't just pop up in the 300's like a fart in the wind, they came from some where and built up into their numbers.