If 'ga' is can or may then doesn't this translate word for word into ' can I orange juice"? Where is the verb?
Ga i...? is the verb. Here it means 'May I have...?'
Literal, word-for-word translations are not always very useful with Welsh because its structures are often quite different from those in English and related languages.
'Ga i' is the future tense of 'cael' =to get, have, obtain'; so literally it means 'Will I get/have/obtain?'
In English this approximates to 'May I have'
With a noun eg 'Ga i oren', 'May I have an orange'
With a verb eg 'Ga i fynd', 'May I have going' or in other words 'May I go'