"Our elephants are not white."
Translation:I nostri elefanti non sono bianchi.
No - "gli" is only used instead of "i" when the letter IMMEDIATELY after the article is a vowel, 'z', or 's+consonant'
In your sentence the letter immediately after the article is the letter "n" - therefore the normal article is used.
hover over 'our' and 'gli' is shown as an option [but not accepted]; hover over elephants, and 'elefante' is shown as plural and 'elefanti' as singular. Am I missing something here?