What does Pendragon mean?
Middle Welsh epithet, pen 'head, chief' + dragon (draig), literally 'chief dragon' / 'dragon's head'. Flag: this is a byname/title (famously Uther Pendragon in Arthurian legend, where Geoffrey of Monmouth glosses it 'dragon's head'), not an ordinary personal name; some scholars read draig here as 'warrior/leader' rather than the beast, so it may originally have meant 'chief of warriors'.
What does Gwilym mean?
Gwilym is the Welsh form of William, “resolute protector.”