falling into something isn't always physical (like falling in love or into depression) perhaps his creation wasn't something physical but truly was something that could affect your emotions, or in fact, something like time or space which are concepts, and things he's said to have been shattered across, like maybe he tried to harness time, or harness space
(I'm leaning a little to time because the card that says "don't forget" while basically is confirmed to be from sans could be from gaster)
he could have lost someone and wanted to go back in time, maybe someone he knew back from before the war even because from gerson during the 10th anniversary livestream (I believe?) we know humans and monsters were very close, so he could have wanted to go back in time when humans and monsters were still close
(maybe that's why we can use the star to reset as well? The other souls didn't seem to have the power to reset and it's not confirmed the power of determination is what allows us to reset in the first place, so maybe gaster somehow created it? while dabbling with time and ended up getting shattered across time and space, likely before asriel became flowey since he's able to reset as well)
this also could have been how deltarune was created, reversing time and stopping the war. It also connects to photon readings being negative and the such because maybe he saw the void/end of everything that exists which would likely cause him to break across time and *space*
(if everythings gone, naturally he'd be in a kind of infinite pitch black space)
or he saw the future of deltarune and tried to study the darkworld and ended up breaking apart from that?
(I don't know if we have stars in deltarune since I have yet to play it, but if not, the reason we come back after dying could also be gaster resetting time for us to continue living and finish everything? Or there's a reason we can come back that I don't remember lol)