So we know gaster made at least a few things, It's a definite that he built the core, and it's at least hinted that he's the one who originally design the DT extraction machine (even if Alphys is the one to build it, a feat in and of itself if she can translate his language.)

But what did he fall into? The core? The DT machine (if he built one before alphys) the machine in Sans' lab?

What do you all think it is?

As for me, i'd like to make a wild theory.

Gaster mentions something in his now famous lab entry:

"DARK DARKER YET DARKER

THE DARKNESS KEEPS GROWING

THE SHADOWS CUTTING DEEPER

PHOTON READINGS NEGATIVE"

Wherever and whatever Gaster was doing in this experiment, there was NO LIGHT of any kind, and it seems he had a device of some sort to tell him this. Beyond this though, that means his own body must not have been radiating heat of anykind, because the only places where NO photons, to my elementary understanding, could exist, Is quite literally a place experiencing a temperature in absolute zero, a temperature we should theoretically not be able to achieve. A photon is an excitement of an electromagnetic field, so we can assume where he is, there isn't a field of that sort there, which presents a problem, as we can play through all available timelines. the only one that ends in nothing, is the genocide route.

So, my theory is, Gaster, in an attempt to see why all these timelines were ending there, teleported or transported himself to or beyond the Genocide timeline with some device, and, since there doesn't exist anything BEYOND that timeline, Gaster no longer exists.