"Gareth is the youngest."
Translation:Gareth yw'r ifanca.
I answered "mae Gareth yw'r ifanca" and was marked wrong. I think it should also be accepted as correct, but I'm not sure
What you've done is included two verbs for "is" i.e "Mae" and "Yw". For this sort of sentence we use the emphatic construction so we use "Yw" (or "Ydy") not mae and the "yw/ydy" comes after the subject.