Gaster or W.D. Gaster is kind of an easter egg you can find in Undertale if you mess up with it's fun value. Gaster is formerly the Royal Scientist but because there was a mistake in his experiments, he disappeared without a trace, leaving no evidence of his existence. Riverman/Riverwoman tells you this: "Beware of the man who speaks in hands." which refers to Gaster. There's also an encounter of having a wrong number in some part in Snowdin. There are some theories that Gaster is probably Sans and Papyrus' father. :3

Hope that explains well. If you got some questions, there's a lot of theory about Gaster himself :3