First image is what Monster Kid says to Undyne if you let them fall–Undyne leaps down to save them, of course.
Second image is Entry 14 of Alphys’ journal that talks about the monsters who had “fallen down” and subsequently came back to life after being injected with determination. It’s important to note that the game never clarifies what “falling down” really entails, though Alphys states in Entry 6 that the bodies of monsters who have “fallen down” are preserved in a comatose-like state before eventually turning to dust.
Both instances involve monsters “falling down”– both instances evoke the phrase “goner”.
The Grey NPC version of Monster Kid that’s associated with the Gaster events is likewise named “spr_mkid_goner”.
I don’t have any theories to tie into this observation. It’s just a connection to consider.
If not, make sure you’ve seen everything first. Because otherwise, anything relating to Gaster is an omega-class spoiler.
This isn’t about who Gaster is or what he does, here I’m just going through what we know to try and reason out what he looks like. Ready, then?
There are four grey NPCs in the game, which are intended to potentially be found after the final fight, while you’re having your victory walkabout in a true pacifist run. Three of them talk about Gaster, and the fourth is named mkid_goner. (Find out more information about them elsewhere, just note that they exist for now.)
We know that Dr. W.D. Gaster is probably named after the fonts Wing Ding and Aster. This makes him related to Papyrus and Sans, therefore we can imagine a skeleton in a labcoat.
We know that Gaster worked with Sans, who inherited some of his machines, and indirectly with Alphys, who inherited his lab and research notes. His devices are still around.
One of the attacks Sans uses against you in a Genocide run, the one which shoots laser beams, is notably named the “Gaster Blaster” internally. (Here’s a reminder.) The same shape is used for the DT extraction machine in True Lab. (If they don’t seem similar enough at first, remember that the Gaster Blaster’s mouth opens sideways, even though that screenshot doesn’t look like it.) Oddly enough, both machines resemble a skull of sorts. Did Gaster have a preference for making beautiful machines in his own visage? Knowing how vain Papyrus is, it seems likely – and along with the general tone of the game, they were probably intended to not seem nearly as scary, since they’d match the friendly face of your beloved Royal Scientist, you know? But those aren’t humanskulls. What kind of skulls are they?
The last attack used by Asriel Dreemurr before they shift forms is called Hyper Goner. It looks like this. This is also majorly skull-like, but obviously still a fully intact face. Does it seem similar? If you take the ears and horns away, leaving behind only the skull, you get… The same face as Gaster. This is a bit of a stretch, but remember those grey NPCs earlier? Also named goner.
This applies to Flowey’s neutral final boss form as well, with the sideways human teeth forming a Gaster Blaster and DT Extraction-like sideways-opening mouth, and since we find out Flowey is a goat, this further asserts that Gaster’s head is like a goat skull.
Therefore… Dr. W.D. Gaster was probably the largest of the skeletons, nearer the size of Toriel, as a gentle Royal Goat Scientist skeleton in a lab coat, with two little assistants. If you were to fight him, there would likely be an unusual mix of falling fire magic and bones to start with, followed by powerful blaster lasers when he gets desperate.
Alternatively: Gyftrot has a head like the Gaster face, so it’s entirely possible that Gaster is a 4 legged creature. Therefore, him having two assistants makes sense, since he’d only be able to be the brains behind the operation, requiring help from others to build and work his brilliant machines.
On the other hand… One interesting tidbit that no one seems to note is that Papyrus and Sans weren’t always there. According to the bunny shopkeep in Snowedin, they just sort of… Showed up one day. It’s entirely possible that… Well… I’ll leave that up to your imagination. That level of speculation is beyond the scope of this thread, but please bear it in mind.
By the way, Mew Mew Kissy Cutie 2 is way better than the original, it establishes changes that were needed to keep the series going, because Mew Mew Kissy Cutie was just too much of a closed-box with all the loose ends neatly tied up by the end, they never expected to get a sequel, so…”