Recently I've thought of another theory.

Now, in the game you can find the Mysteryman whom everyone presumes to be Gaster, however this one does not speak.

Then we have that weird white ghostly thing that speaks in Wingdings, like W.D. Gaster does according to his entry nr. 17. But this one doesn't look like anything.

Then we have a weird talking head held by one of Gaster's Followers.

And then we have Gaster's Entry Number 17, in which he ends with "What do you two think?"

Now, many people think that he's referring to Sans and Papyrus, however the wiki suggests he may instead be referring to Sans and Alphys, which would make Gaster the missing link as to how the two of them know each other, as said in the TP ending.

So if the last part is true, then what about Papyrus? And this is where my new "theory" of sorts come in.

The Gaster Follower who holds the talking head says that he "scattered across time and space" and "I'm holding a piece of him right here".

Which means Gaster may not be in one piece. So, what if BOTH Mysteryman AND the ghost thing are Gaster? Mysteryman could be a representation of his former body, while the white thing is a representation of his mind/SOUL with his speech function. I dunno what the talking head would be though.

Anyway, that's my headcanon for the canon Gaster. Now for the more farfetched part of my theory.

Gaster could have split into three main parts: his Mind, Body and SOUL, or SOUL, astral body and physical body.

His SOUL or mind would be that white ghost thing that speaks Wingdings. His former body or astral body would be the Mysteryman, who also seems to be an amalgam, possibly.

And his physical body, the one he has been ripped from, that may be Papyrus. Which in turn only has his goodness and purity in it, and no memory of his former self.

The reason I think it would only be Papyrus and not him and Sans is because of Entry Number 17 "What do you two think?"

If Gaster is referring to Sans and/Papyrus, then at least Sans must have existed while Gaster did. In other words, he wouldn't have split into Sans. But he COULD have split into Papyrus, as the other one could be Alphys instead of Papyrus.

As no one remembers Gaster, Sans would likely then believe that Papyrus has always been his brother, or he even knows Papyrus was once Gaster but doesn't talk about it.

I know this is a very farfetched theory and probably not true, but I still found it interesting.

I still believe the first part of my theory is true, the second part with Papyrus is more speculation.