The short answer is that 'g' isn't changing it is 'ん' coming before either a 'g' or 'k' sound. This is why 日本語 has a nasally 'ん' and it sounds like a double 'g'. There is a bunch of rules for the sound of 'ん'. It depends what sound comes after it. Basically, it has 4 different sounds. Not completely different but considerably. It is either 'n', 'N', 'ng' or 'm'('m' is aruguably 'n'. It's just that it sounds like a 'm' when it comes before 'm','p', or 'b').

Yukigami, what you say is true but you're thinking of something else.

We're not talking about the "n" before -t, -k, -m and +V (morpheme boundary plus vowel), all of which are different, but the pronunciation of "g" between vowels.