According to the guidelines in the book called “The Pronunciation of Japanese” written by Dr Cai, the rules of Nasal Sonant (P.54) are as follows:
In fact, there are 2 kinds of pronunciations of the "row Ga"（ガ行）. The first is called a voiced consonant of the "row Ga". The second is called a nasal sonant of the "row Ga". If the first syllable of a word belongs to the "row Ga", then pronounce a voiced consonant; if the second or third (so on and so forth) syllable of a word belongs to a kana of the "row Ga", then pronounce a nasal sonant.