"Pryd maen nhw'n mynd i'r gwely?"

Translation: When do they go to bed?

[Chlax7]

Why is it 'to the bed'? Is this just to avoid using a mutation on 'gwely' or is it grammatically correct to use the definite here?

[EllisVaughan]

Grammatically correct. We use the definite article a lot in Welsh where it isn't used in English.