Query re feminine form of an adjective

I have been listening to a very beautiful Welsh song called ‘Myn Mair’ and think I have it all sussed out except for one phrase ‘yn gref’, as in ‘A Mair, Mam y Duwdod, eiriolwch yn gref’. I think this ‘gref’ must be the mutated feminine form of ‘cryf’ meaning ‘strong’, but why the feminine form here? – is this because Mair is female so the gender rule applies even though it looks as if the adjective is just being used adverbially rather than applied directly to Mary? But if you were appealing to, say, a male saint to intercede it would be ‘yn gryf’? And you would say ‘Mae’r bachgen yn rhedeg yn gryf’ but ‘Mae’r ferch yn rhedeg yn gref’ (assuming you would use cryf in this way at all).
And just out of interest, are these feminine forms of adjectives still strictly adhered to in these gender-neutral days? You would definitely always say ‘Y bachgen cryf’ but ‘y ferch gref’?

Yes, you’re right, cref is the feminine form of cryf.

Bear in mind that you’re talking about a song - although you may well hear some people using a feminine form in speech, (I’d say they are often adhered to rather than strictly adhered to) they are altogether more common in songs (especially older/hymn/folk ones) and poetry.