As a native spanish speaker, i could tell you in spanish the "GN" is not pronounced "NY". This happens mostly in the italian pronunciation. In spanish we have a specific letter for that sound. The "Ñ". Also, is widely accepted in ecclesiastical latin that "GN" is pronounced "NY", same as the spanish "Ñ".
in any case, it seems that the NI is very well established: I've just watched/listen to 14 interpretations of the Agnus Dei on youtube, only one of them was pronouncing GN. 😉🤓
You have definitely no idea of how Spanish is pronounced. "Gn" doesn't even exist in Spanish. It is ñ. You mix it with French and Italian in which the gn is pronounced nyy
