Board Thread:General Discussion/@comment-26006155-20190617222636/@comment-32182236-20190726113307

I actually think I have an answer.

Recall Kingdom Hearts for a moment, and how Roxas lost his memories as Sora.

...But they weren't really gone. It was scattered and rearranged by Naminé, so that it couldn't be recalled. She was eventually able to recover them back to how they were before, to give Sora his memories back.

We know Gaster was actually scattered across time and space.. What if his memories were scattered with him?

After all... one of the followers are holding a piece of him right now...

If his memories are connected to him... It could serve as an alternative explanation for why Sans remembers Gaster.. If Sans's existence is a result of Gaster's.

Not everybody forgot Gaster. Some remembered, and were lost alongside Gaster. Remember, the Followers remember him just fine, but appear to be in a reality separate from everyone else... That or they jumble up the phone.

So I don't think his memories were truly lost. Just scattered across time and space, and thus nearly impossible to piece together.