"Dim ond munud gyda fi" is wrong?
Yes. Your phrase has no verb.
Thank you that is a simple, straightforward answer.
Is 'dim ond un munud sy gyda fi' really wrong?
I struggled with that and finally figured out that I was equating "a" with "one" , which I suspect is not a Welsh thing.
Why is it gen i but gyda fi?
When it is used in front of a word starting with a bowel, gyda usually adds a -g:
However, when it is used with the pronouns i and e/o, the pronouns themselves change to their forms fi and fe/fo instead:
"Dim ond un munud sy gen i" should be accepted; the "un" would emfasize that it is only one minute.