Is it weird that we're using Gallu/Medru here, rather than Cael?
I gather "Cael" has strong overtones of "permission". I mean, I've seen English-speaking pedants answer "Can I..." questions (where permission is clearly intended, rather than ability) with "I don't know - can you?" as a sarcastic rebuke for not choosing "May I..." instead.
There are subtle differences in meaning.
In general and informal use 'Ga i' is the usual form used for this type of question.
However in a more formal environment, where maybe competence is the context, forms of 'gallu' may be preferred, eg in a lecture where you're unsure if you know enough to interrupt the lecturer.
Then either of the following may be used:-
Alla i ofyn cwestiwn? = Can I ask a question? (This is often contracted to 'allai ofyn cwestiwn')
or less common
Allwn i ofyn cwestiwn? = Could I ask a question?
See rmcode's answer above:
- If you are asking permission, use Ga i...? - May I...?
- If you asking if you have the ability to do something it is Allwn/Fedrwn i...? - Could I ...?