Ego, I hear lot of newscasters realizing /g/ as [ŋ] word-medially. It seems like it's a characteristic of formal spoken Japanese. In casual speech /g/ is sometimes realized as a voiced velar fricative, like our γάμμα.
But /n/? It seems there's a palatal allophone (に＝[nji], not [ni]) but not a velar one. Of course, syllabic ん (what's the right IPA symbol btw?) is [ŋ] before /k/ and /g/. Maybe the underlying word wasn't この but こんご: /koんgo/->/koŋgo/->[koŋo]? This is a feature of some Japanese dialects (I read it some time ago) but I don't remember which.