Isn't it very, very interesting that Mystery Man and Redacted are only possible theoretical gaster, while this guy is a confirmed in-game gaster
wait, who dat?
He’s held by Gaster follower 2. Although technically, he’s said to be a piece of Gaster, not Gaster himself.
its gasters head-thing on their pirate ship
LETS GET DOWN TO BUSINESS
i dont know the song at all T-T
Ah, I don't believe that...
You may think this theory is crazy...
but...
I believe that Gaster split in three, and became Mystery Man, Sans, and The voice
Sans has an attack called "Gaster Blaster" and his workshop has blueprints with unreadable symbols.
We don't know too much about the other 2...
BUT...
in entry 17 in Room_Gaster, it says "What do you two think", referring to three (Whoever said it and the other 2)...
and I think that The Voice and MM are opposant mental sides of Gaster, and they got in a fight...
Sans (Gaster's Power) had created a Dark Fountain (Photon Readings Negative) and got rid of the voice. MM and Sans then fought and separated...
Finally, the Gaster Followers had seen Gaster split in 3, mentally and physically affecting them.
Here you go https://youtu.be/liRlUQFbkiI
Fire theory 
But also why did I skim through the discussion where you first thought of this literally five minutes ago
I just felt this needed to be mentioned...
As some people think that the Gaster Followers are Gaster (I'm not saying that they're wrong)
and need this perspective.
No, just no please
Can't it be [[REDACTED]]
No, Redacted just seems more similar too a Gaster Follower
Okay then
Sans absolutely isn't Gaster though
Maybe related but not the G. Man himself
AND VALUE, VALUE THE FUN!
BRUSH THOSE THEORIES OFF NOW, OUR MYSTERY MYSTERY MAN
I say he isn't Gaster himself, just what was left of his possible powers.
THOSE PLAYERS NEVER TREAT ME RIGHT, THEY HAVEN’T SEEN
Just go here: Gaster split
MINOR SPELLING MISTAKE
WHAT I TRULY MEANT IN MY OLD ENTRY SEVENTEEN!!
AYE, WE PLUNDER ROOT BEER, AND OF COURSE WE PLUNDER ALE!