Board Thread:Questions and Answers/@comment-25958552-20160825130425/@comment-27701762-20160825145321

It's unlikely. It is never stated when the Core (one of Gaster's creations) was built, only that it took a fairly long amount of time until Asgore hired Alphys as the new Royal Scientist. So if the Core was constructed around the time of 201X, then perhaps Chara met Gaster. If it was constructed later, theoretically any of the other humans could have met Gaster, though there is no indication of this since the humans seemed to die in very different places.

In short, there's just no evidence of any of the previous humans knowing Gaster. There are no explicit statements, or even tiny clues that would suggest it. In the absence of any evidence, the default conclusion is that none of them met Gaster.

As for Alphys, she never mentions meeting any other humans at any point in her life, which would suggest that none of the humans met her.