WARNING The following post contains major spoilers, so if you haven't finished the game and don't enjoy having the fun sucked out of a game, do not proceed.

So, to start off, let's recall some confirmed things we know about the plot-driven mechanics of Undertale as a whole.

• It's implied that the power to SAVE and LOAD is greatly affixed to least high Determination, which is most commonly achieved through possession of a Human SOUL. Flowey is only able to SAVE and LOAD via the Determination he received from Alphys.

• It's also implied that SAVEing and LOADing results in “timelines jumping left and right, stopping and starting” from Sans' fight dialogue. In addition, it's also stated that when a player SAVEs and LOADs, the characters for the most part do not recall any of the events save for very brief and vague memories (akin to Deja Vu)

• Alphys was working on a way to extract Determination from the Human Souls using the DT Extraction Machine (which ironically acronyms itself to DTEM, or "DTEM"-ination). The Determination she extracted created the Amalgamates, and ressurected Asriel in the form of Flowey.

• Gaster, being the previous Royal Scientist, created the Core (as stated by the Grey NPC Followers), as well as supposedly being a scientist who was "an act to follow!"

• Sans has clear knowledge of (or at least a sharp awareness of) the Player's ability to SAVE and LOAD. This is clearly shown in his Genocide Route fight, and in his dialogue leading up to his lab in the basement.

• Sans is also supposedly related or connected in some way to Gaster, as in the lab there are blueprints with notes written in symbols (believed to be Wingdings), as well as a Broken Machine, and a photo album of Sans and "people you don't recognize".

• Sans is also capable of "teleportation", or at least of some form of phasing. This is referenced by Papyrus in multiple quotes, and in Sans' ability to man 4 sentry posts at the same time, and through his "shortcuts".

• It's unconfirmed and unreferenced, but also still chronologically viable that Gaster was alive at the same as the other Six Humans (which would give him access to their SOUL), as an official timeline of these events is not revealed.

• Sans, Asgore and Gerson have both referenced that they are aware that they are part of a video game without access to SAVE and LOAD (in the Dirty Hacker ending and Genocide Boss fight, the Asgore Boss Fight through destroying the Mercy button, and in the Genocide Route Shop Dialogue, respectively). Flowey and Chara also appear to have knowledge that they are part of a game, however through possession of a Human soul, they are expected to know that, and are somewhat excluded from this point.

Now then, putting this information together in a really poor fashion after reading the two Bad Time Machine posts, I came up with this. Based on Sans' teleportation/shortcutting, and knowledge of SAVE and LOAD, it can be said that Sans himself is aware of how to SAVE and LOAD, but cannot do it himself due to a lack of a SOUL. Now taking into fact that Sans is likely to have either worked with Gaster, or studied under him (seen in the blueprint in his workshop), it could be inferred that Gaster at some point studied the concept of SAVE and LOAD, and passed on his notes and knowledge to Sans.

Now it's clear that Gaster's most well known device was the Core, along with Sans' Gaster Blasters. But, assuming that Gaster had at some point worked on SAVE and LOAD, possibly at a time where access to the Six Souls was possible, it could be relatively possible that Gaster at some point attempted to utilize the power of the Souls to create a device that could SAVE and LOAD, or at least enable him to.

Of course, we know from his "Followers" (the three Grey NPCs) that his life was cut short by him falling into one of his creations. Some people infer this creation to be the Core, however, it is much more plausible, plot wise, that the "Creation" he fell into is the SAVE Device he had created. Theoretically, he would've fallen into the device, which could have possibly been incomplete, and was SAVEd incorrectly, resulting in a corrupt save file. This would essentially, gameplay wise (and as a major breach of the fourth wall), render him incapable of being accessed to as a character except via data manipulation (which is also, ironically the only way you really become aware of his existance). This corrupt SAVE file would mean he could not be LOADed, and cannot be re SAVEd, meaning he would be stuck as a piece of irrelevant code that doesn't do anything and just sort of sits there, experiencing the game as the code itself, being present everywhere at the same time. This could be why some parts of the game that are suspected of being related to him state [Redacted] when interacted with, as the code that the game would've been redirected to when triggering that event is now either gone, or unreadable.

Now onto the Physics stuff. Sans references that SAVEing and LOADing messes with timelines on the Time-Space Continuum, which in itself is really high level physics. In retrospect to that level of physics, an abstract way of stating what Gaster did would be saying he removed himself from reality and existance, in terms of the game. This would make sense why for the most part, none of the characters (save for six of them; Sans, the River Boat Man, the Grey version of the Monster Kid, and the three "Followers") seem to remember or make any reference of Gaster in general. There are no references to him in the True Lab (not accounting for Entry 17, itself an entry by Gaster in Wingdings), nor are there any mentions of him in any History Books in the Library, or in any non-Follower dialogue, not counting the River Boat Man, who is supposedly omniscient.

Now the Machine that Gaster had fallen victim to could very well possibly be the Broken Machine in Sans' workshop. Another reason why Sans may have a photo album with "People you don't recognize" and Gaster's blueprints is as a form of remembering Gaster. Had Gaster actually removed himself from Spacetime via a corrupted save file, Sans would not be able to remember him both theorhetically as a person, and as a program as a game, because Gaster would have ceased to exist at that point, and because Gaster's data would've become corrupted and unreadable, respectively. This creates a weird, morbid theme behind Gaster, if Toby Fox intentionally crafted Gaster this way as a person who fell victim to a corrupt SAVE file. In addition, Toby Fox did tweet that the Broken Machine would remain broken. This may be because Gaster is permanently gone, and cannot be recovered. In addition, it would appear that a better, fully operational platform is now present for the process of SAVEing and LOADing, although it is limited to Frisk (and to some extent, Flowey and the Annoying Dog). This platform is the Undertale game itself. This could also answer why Gaster is constantly aware of your actions, regardless of resets, as he is now permanently part of the game, literally.