How to actually get Gaster, Redacted (assuming they're connected, despite the wingdings font it's just a sprite blob connected to an entirely unused event and is just a stretched version of parts of Muffet's battle sprite), or his various grey NPCs to show up is a mystery. I had heard a week or two ago that Fun becomes modified naturally after killing Sans in a Hog Wild run but I never actually looked into if this was a hoax or not myself. That would make sense from a narrative standpoint as Sans' laser attack is called a Gaster Blaster, and considering both are skeletons they're probably related and Gaster would probably try to come back for revenge or whatever. But for such a character who seems to be based upon games' unused content and creepypastas, I don't feel that that really fits?

My theory is that Gaster is supposed to be found this way. Recently, in the Korean demo (which has significantly more of an earlier version left inside it than the English version), someone found out that it was a character named Grandpa Semi that was supposed to give you the pizza call on your phone instead of Alphys. More digging into the files of that demo revealed a grandpasemi.ogg that was the first notes of Metal Crusher, Mettaton's first battle theme. A friend and I got talking about this and thought, hey, maybe Gaster was inspired by this character's removal?

Semi is obviously short for Semi Serif, a font, so he was probably a skeleton and probably Sans and Papyrus' grandfather. With the only remnants of him left being another version of a call from the scientist in the final game and parts of said scientist's robot's theme as his theme, it's only natural to suspect that at this point in development, he was the royal scientist. For whatever reason, he was replaced with Alphys.

Now, can you see what I'm getting at? Gaster is a character based on cut content from games, literally. In the current build, he fills the exact same role that Semi filled during development: a skeleton relative and scientist removed from the world and replaced by Alphys. I can easily see this being a way to turn another normal part of games into a character or statement, except instead of being a statement on EXP or LV, it deals with how the characters and world itself react to something just going unused. They know that a previous scientist built the core, but don't remember his name. The grey NPCs are the only ones who call him by name and know what happened to him: that he fell into the core and shattered across time and space. The only "normal" people who seem to remember him are the ferry owner ("Beware the man who speaks in hands") and the mysterious caller who has the wrong number ("Hello, I'm looking for G..."), and for all we know are both grey npcs as we never actually see what they look like. The only time someone would even see a grey NPC in the first place would be if their Fun stat changed, and the triggers for that honestly seem to be random and unexpected. He leaks back into the game, but never for long. I expect a future patch to add a lot more to his story.

Outside of his real-world origins, there's also the matter of how he'll fit into the plot. I believe Sans knows about him, and may have even worked for him as an assistant. We know from Entry 17, Gaster's spooky entry that isn't in the True Lab but rather prints across a black void in wingdings, that he spoke to two people ("What do you two think?"). We know one is probably Alphys, as she probably was picked because she was next in line, but the other? Sans has an interest in quantum physics, Alphys mentions knowing him in one of the dialogues after defeating Asriel, he mentions being able to monitor timelines, he teleports around, and he has a big, mysterious machine in his house. He clearly has some sort of scientific background. Not to mention that in the fight against him, his eye flashes between two colors and isn't just blue. Blue usually means no movement or the color a skeleton turns your soul while fighting you, but his eye also turns yellow. The only time your soul turns yellow is when you're fighting against Mettaton and Alphys, who are definitely both science-related. Finally, Sans can have two pictures in his secret lab. One is if you do not stay with Toriel in the pacifist ending, but the other is always there and has features you (whether this means Chara or Frisk is up in the air), Sans, and "people you don't recognize". I believe this photo is actually from an "early timeline" (aka early phase of development) when Semi/Gaster still existed. Sans probably managed to take this picture from that timeline using his machine and seeing it allows him to have his ability to remember things from past timelines. So whenever Gaster is officially revealed (assuming that hiding him like this wasn't the intended method to do so), Sans will probably be heavily involved in the plot.

Honestly though, considering the goofy natures of the two remaining skeletons? I would not be surprised in the slightest if you get him back in this theoretical future story patch and he just goes, "Man, I thought I was boned." and nothing is ever explained, the grey NPCs disappear, and he just hangs out in Snowdin giving out bad puns in a font you can't read. It would be fitting for this game, especially after that True Lab bed scene.