"Ga i losin?"
Translation:May I have some sweets?
'Wedi' is used before a verb to express the perfect tense in English. To show a completed action.
I have gone = Dw i wedi mynd
I have come = Dw i wedi dod
'Ga i' is used to express possessing something, the use of 'have' in English to show possession.
Ga i afal? = May I have an apple?
Ga i siocled? = May I have chocolate?
This is very well explained in the notes for this section, available on the web based Duolingo course.