The grey head that that one of the followers holds is the canon way that Gaster looks
And the Mystery Man is probably a Uboa reference, since toby is known to have been a fan of that easter egg, and the resemblance is uncanny
So why is everybody making fanart of a Uboa callback instead of the canon piece
Seriously at least put a picture of the grey head on Gaster's page, it's like even the wiki prefers Mystery Man to something canonically referred to as a piece of Gaster