の, が, and ゲ in place names

Hi!

I'm a lapsed upper-intermediate student of Japanese

who's been haunted for the last few months by a quirk (or is it?) of spelling in Japanese:

as in "Awazugahara" in this performance of Tomoe—the place is spelled with a rōmaji "ga," pronounced in this performance with a の, and probably spelled with a ゲ sometimes.

Please, whoever knows—what's up with that? Is it just a 'Worcestershire'- or 'Cholmondeley'-style vestigial spelling?

Thank you!