'Sipendi kanzu' is a general statement implying that the speaker, generally, does not like gowns. The current required answer: 'I don't like the robe' is therefore incorrect since, in English, the article 'the' introduces specificity.
'Siipendi hiyo/hii kanzu' - I don't like the robe or I don't like that/this robe. Notice I used 'Siipendi' rather than 'Sipendi' because in speech it seems to be a double 'i' rather than a single one.
I think "I don't like the gown" is also correct, as in Swahili people normally don't use a word to indicate "a" or "the".
Considering Wikipedia describes the Kanzu as "referred to as a tunic in English", I feel tunic should be a valid translation of kanzu.