from what i understand, dr. w.d. gaster (wingdings aster?? both of those are font faces, incidentally) was the royal scientist before alphys--super smart dude, created the core and possibly the broken time machine in the skelebros' house. but after an accident he ended up 'falling into one of his creations' (maybe the core, maybe the time machine), and he was apparently torn apart across the timespace continuum

on a meta level he's literally scattered in parts of the game's code that are either impossible or very hard/unlikely to access through normal gameplay--almost all gaster references have been found by mining the code or manipulating certain code values until an event is triggered. there's dummied-out npcs in the game that can be triggered into spawning and talking; some people think these are gaster's colleagues/pieces of gaster himself. one can also trigger a bundle of wingdings dialogue that translates to the missing 17th entry from the true lab. https://www.youtube.com/watch?v=LwAWrrmfoJs here's a compilation of (some of?) the gaster events, alongside other dummied-out odds and ends of the game

all in all it's a nifty easter egg at the very least, and might have some traceable story to it too; the font thing makes people think that gaster might be related to the other two skelebros. otherwise it's all very 2spooky and likely one big shoutout to things like yume nikki, but it's neat all the same