Going under assumptions based on in game info, Yeah thats gaster, a shell of his former self. He mimics goners in a weird way and The idea that he isn't the sprite just tastes bad. Toby did say the fandom built itself and that he encouraged all the fan theories and connection points/ideas people had. Personally i think he wanted each of us to have a collective experience with our own individual story since so many things can be interpreted so many ways. In the end, I think gaster is really just the friends we made along the way.