@Alone-zee Ideally one can eventually get beyond the /nj/ approximation to /ɲ/ itself. Since that nasal is palatal it's inevitable that you articulate (or perceive, in any case) the palatal off-glide /j/ when transitioning to another vowel. If the question comes down to whether <gn> may ever be approximated /n/, or always /nj/, the answer is always /nj/ since that's closer to its true pronunciation /ɲ/.

I don't know if there are any critical pairs in French (probably not since there are hardly any -nio-), but I know there are many in Italian. I remember the first time someone tried to show me the difference. I really couldn't tell.

Ah wait, maybe there are with conjugated verb forms.

Ok, avec une petite recherche rapide avec mon dico j'en trouve 8 : manions, minions, nions, peinions, chinions, canions, plainions, résinions. Plus toutes les autres conjugaisons similaires aux autres personnes ou aux autres temps (pour le clash magner/manier). Et magna/mania, ligner/linier (connaissais pas), niôle/gnôle (qui est le même mot… ça veut tout dire). Le dernier cas que j'ai trouvé avec l'orthographe, vernie/vergne, n'est pas une paire critique.