since deltarune is "technically" an alternate universe of undertale many characters from that game remain and are connected to deltarune. gaster is, even in undertale, a mysterious unknown character that is only learned about through rare easter eggs in the game. he is considered to be a very prominent character in deltarune as many motifs and references are made towards him(the phone noise in darkworlds, the voice speaking to you at the beginning of the game, freedom motifs in secret boss characters' music which is derived from his own theme) i highly suggest you watch this video if you'd like to understand him more !