TL;DR: Officially, no. But it depends on what you think. There seems to be a lot of confusion on whatever we know for sure about this character. Let me clear things up by making a list of what we know for sure. read more
Gaster looks vaguely skeletal from what little we see of him, and his initials W.D. matching his font WingDings, hints at a relation to Sans and Papyrus. Also, he addresses two unknown characters in Entry 17, possibly Sans and Papyrus. read more