This isn't a perfect theory but I'm gonna throw this out. What if the identity of Gaster is: another irl player of the game. You can never input his name because someone else already made it their own, he was Assgore's top scientist because he was programming shit into the game for him, being "shattered accross time and space" could simply mean he started playing other games, "it's rude talking about someone who is listening" could be reffering to the fact that he's reading the dialouge just like the current player is.