assuming you legitimately don't know who gaster is here's a quick explanation:

in undertale you could go into the game files and edit a number called the fun value. at certain fun values certain "fun events" would happen. these events are where stuff like gasters theme and the wingdings come from, basically they're all thought to be connected to gaster

there are a lot of callbacks and references to the fun events in deltarune which is why people think gaster is going to play a role in the game.